Exhibit 99.1

National Interstate Corporation Reports 2012 Third Quarter Results

Richfield, Ohio, October 31, 2012 – National Interstate Corporation (Nasdaq: NATL) today reported results for the 2012 third quarter and first nine months. Net income per share, diluted of $0.46 for the 2012 third quarter increased compared to the 2011 third quarter reflecting improved underwriting and investment results. Net income per share, diluted of $1.33 for the 2012 first nine months also increased compared to 2011 as a result of higher investment income that was offset by lower underwriting profits. In addition, net income for the 2011 first nine months was adversely impacted by the balance sheet guaranty related to the 2010 Vanliner acquisition.

Gross premiums written of $126.3 million for the 2012 third quarter and $416.6 million for the 2012 first nine months increased 4.5% and 1.2%, respectively, compared to the same periods last year.

Earnings

Net income, determined in accordance with U.S. generally accepted accounting principles (GAAP), includes items that may not be indicative of our ongoing operations. The following table identifies such items and reconciles net income to net income from operations, a non-GAAP financial measure that we believe is a useful tool for investors and analysts in analyzing ongoing operating trends.

Net after-tax earnings from operations include underwriting income and net investment income. After-tax realized gains from investments and the after-tax impact on underwriting results related to the balance sheet guaranty from the Vanliner acquisition are separately presented to better reflect the results related to ongoing business.

Underwriting Results:

Dave Michelson, President and Chief Executive Officer said, “Our underwriting results for the 2012 third quarter were better than both the 2011 third quarter and the first two quarters of 2012. This improvement is noteworthy given that the claims results for the third quarter have historically been elevated when compared to other quarters of the year. We are encouraged by reduced claims severity in our higher limit passenger transportation products and it appears we may be turning the corner on several products that have been performing below our standards.”

These underwriting ratios exclude the impact of the runoff of the guaranteed Vanliner business based on premiums earned of $1.5 million and $25.0 million for the 2011 third quarter and first nine months, respectively. There were no such premiums earned in 2012.

Claims: The loss and LAE ratio for the 2012 third quarter of 73.4% was 1.0 percentage point better than the 2011 third quarter and in line with the claims results for the previous two quarters of 2012. During the 2012 third quarter several products in the specialty personal lines and alternative risk transfer (ART) components had improved claims results. In addition, the Company experienced fewer large claims in the passenger transportation products which favorably impacted the 2012 third quarter claims results. The loss and LAE ratio for the 2012 first nine months of 73.8% was elevated when compared to 2011 reflecting adverse claims including large claims in traditionally well performing products during the first half of the year.

For the 2012 third quarter the Company had favorable development from prior year claims of $0.1 million which compares to $0.8 million of favorable development for the 2011 third quarter.

Underwriting Expenses: The underwriting expense ratios for the 2012 third quarter and first nine months improved compared to the same periods of 2011 and remained within the expected range. The Company’s quarterly underwriting expense ratios can vary based on the mix of business written. The 2012 third quarter underwriting expense ratio reflected business written with a lower cost structure than that written in the 2011 third quarter.

Investments:

Net investment income of $8.7 million for the 2012 third quarter and $26.8 million for the 2012 first nine months was 14.3% and 20.3% above the same periods last year, respectively. The higher investment income is attributable to the portfolio repositioning that occurred in the last half of 2011 when the Company took advantage of the steep yield curve and volatility in the fixed income sectors to reposition its portfolio into higher yielding investments. As expected, 2012 third quarter net investment income was below the first two quarters of 2012 due to the continued low fixed income yields that are available for new investments.

Approximately 94% of the Company’s fixed income portfolio is rated NAIC 1 or 2 with an effective duration of 3.9 years, which has remained stable over the last several quarters. The relative high quality and short duration of the fixed maturity portfolio and the current low interest rate environment contributed to an unrealized gain of $46.3 million or 4.9% of the amortized cost at September 30, 2012. The fair value and unrealized gains (losses) of fixed maturities and equity securities were as follows:

Gross Premiums Written

Mr. Michelson commented, “As expected, we are seeing a gradual improvement in our top line with a 4.5% increase in the 2012 third quarter which pushed us slightly ahead of last year for the first nine months of the year. Rate increases in our ART and Transportation components are occurring with more regularity and the actions we took last year related to two products in the program business portion of the ART component will have a lesser impact in the 2012 fourth quarter.”

The table below summarizes gross premiums written by business component:

Alternative Risk Transfer: The ART component grew 4.5% for the 2012 third quarter which moved the year-to-date gross premiums written for the component slightly ahead of the first nine months of 2011. The growth in this component was attributable to new customers, an increase in exposures and the continuing very high customer retention. Offsetting the growth was the residual impact of a product that was terminated in 2011 which comprised approximately 18% of the 2011 third quarter and 11% of 2011 first nine months gross premiums written for this component.

Transportation: The Transportation component grew 7.9% in the 2012 third quarter primarily due to growth in truck transportation which saw increases in both rates and exposures, as well as the impact from capitalizing on new business opportunities in the gradually improving commercial insurance market. Gross premiums written for this component increased 4.9% for the first nine months reflecting the truck growth as well as an increase in rates and new customers in the moving and storage product.

Specialty Personal Lines: Gross premiums written in the specialty personal lines component declined 6.2% in the 2012 third quarter, which is consistent with the prior two quarters of this year. For the first nine months of 2012 gross premiums are down 7.9% compared to 2011 as a result of underwriting and pricing actions related to the commercial vehicle product and a trend towards purchasing direct versus through agents for the recreational vehicle product.

Hawaii and Alaska: Gross premiums written for the 2012 third quarter and first nine months were flat compared to the same periods last year, respectively, which is as expected.

Summary Comments

“The third quarter was improved compared to the first half of the year and slightly better that we expected,” stated Mr. Michelson. “Improvement in our top line and underwriting results and continued solid investment performance contributed to a satisfactory quarter. We are seeing rising rates and competitor actions with some regularity in many of our markets which are clear signals that the insurance markets we compete in are becoming more rational.”

Earnings Conference Call

The Company will hold a conference call to discuss the 2012 third quarter results tomorrow, Thursday, November 1, 2012 at 10:00 a.m. Eastern Time. There are two communication modes available to listen to the call. Telephone access to the conference call and Q and A session will be available by dialing (877) 837-3911. Please dial in 5 to 10 minutes

prior to the scheduled starting time. The conference call will be broadcast live over the Internet. To listen to the call via the Internet, access our website at http://invest.natl.com and follow the instructions at the web cast link. The archived web cast will be available shortly after the call on our website.

Forward-Looking Statements

This document, including any information incorporated by reference, contains “forward-looking statements” (within the meaning of the Private Securities Litigation Reform Act of 1995). All statements, trend analyses and other information contained in this press release relative to markets for our products and trends in our operations or financial results, as well as other statements including words such as “may,” “target,” “anticipate,” “believe,” “plan,” “estimate,” “expect,” “intend,” “project,” and other similar expressions, constitute forward-looking statements. We made these statements based on our plans and current analyses of our business and the insurance industry as a whole. We caution that these statements may and often do vary from actual results and the differences between these statements and actual results can be material. Factors that could contribute to these differences include, among other things: general economic conditions, any weaknesses in the financial markets and other factors, including prevailing interest rate levels and stock and credit market performance which may affect or continue to affect (among other things) our ability to sell our products and to collect amounts due to us, our ability to access capital resources and the costs associated with such access to capital and the market value of our investments; our ability to manage our growth strategy, customer response to new products and marketing initiatives; tax law and accounting changes; increasing competition in the sale of our insurance products and services and the retention of existing customers; changes in legal environment; regulatory changes or actions, including those relating to regulation of the sale, underwriting and pricing of insurance products and services and capital requirements; levels of natural catastrophes, terrorist events, incidents of war and other major losses; adequacy of insurance reserves; and availability of reinsurance and ability of reinsurers to pay their obligations. The forward-looking statements herein are made only as of the date of this document. The Company assumes no obligation to publicly update any forward-looking statements.

About National Interstate Corporation

An Insurance Experience Built Around You.

National Interstate Corporation (Nasdaq: NATL), founded in 1989, is the holding company for a specialty property-casualty insurance group which differentiates itself by offering products and services designed to meet the unique needs of niche markets. Products include insurance for passenger, truck, and moving and storage transportation companies, alternative risk transfer, or captive programs for commercial risks, specialty personal lines products focused primarily on recreational vehicle owners and small commercial vehicle accounts, and transportation and general commercial insurance in Hawaii and Alaska. The Company’s insurance subsidiaries, including the three primary insurers, National Interstate Insurance Company, Vanliner Insurance Company and Triumphe Casualty Company, are rated “A” (Excellent) by A.M. Best Company. Headquartered in Richfield, Ohio, National Interstate is an independently operated subsidiary of Great American Insurance Company, a property-casualty subsidiary of American Financial Group, Inc. (NYSE: AFG) (Nasdaq: AFG).

Contact:

Tanya Inama

National Interstate Corporation

877-837-0339

investorrelations@natl.com

www.natl.com

NATIONAL INTERSTATE CORPORATION

SELECTED FINANCIAL DATA

(In thousands, except per share data)