UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the

Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): October 8, 2010

UNITIL CORPORATION

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (603) 772-0775

N/A

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

On October 8, 2010, Unitil Corporation (“Unitil”) entered into the Fourth Amendment Agreement with Bank of America, N.A., as administrative agent, and a syndicate of other lenders party thereto (the “Fourth Amendment Agreement”), further amending the Credit Agreement dated as of November 26, 2008 among Unitil, Bank of America, N.A., as administrative agent, and a syndicate of other lenders party thereto (as amended, the “Credit Facility”) and amending the promissory notes issued thereunder. The Credit Facility was previously amended on January 2, 2009, March 16, 2009, and October 13, 2009 to, among other things, increase the maximum borrowings under the facility to $80 million and extend the scheduled termination date of the facility.

The Fourth Amendment Agreement:

On October 8, 2010, Unitil paid the administrative agent (for the account of the lenders) an upfront fee of $280,000. In addition, Unitil is required to the pay to the administrative agent (for the account of the lenders) an annual commitment fee equal to 0.30% of the average daily amount by which the aggregate commitments exceed the sum of (i) the outstanding amount of committed loans and (ii) the outstanding amount of letter of credit obligations.

Unitil may use the Credit Facility for its working capital needs and to fund interim capital expenditures. Unitil cannot use the Credit Facility for acquisitions.

The Credit Facility contains customary terms and conditions for credit facilities of this type. The Credit Facility includes financial covenants, including, without limitation, covenants restricting Unitil’s ability to incur liens, merge or consolidate with another entity or change its line of business. Further, the Credit Facility contains a covenant restricting Unitil’s ability to permit funded debt (as defined in the Credit Facility) to exceed 65% of its capitalization (as defined in the Credit Facility) until the Credit Facility terminates and all amounts borrowed under the Credit Facility are paid in full.

The events of default under the Credit Facility include, without limitation, (i) Unitil’s failure to pay outstanding principal or interest, (ii) failure of representations or warranties of Unitil (or any of its subsidiaries that becomes a party to any Credit Facility-related documents) to be correct, in any material respects, (iii) failure by Unitil (or any of its subsidiaries that becomes a party to any Credit Facility-related documents) to perform any other term, covenant or agreement if such failure is not remedied within 30 days of notice of such failure, (iv) a cross-default with Unitil’s (or any of its subsidiaries’) other debt in certain circumstances, (v) a change of control of Unitil, (vi) non-appealable judgments in excess of $12.0 million in the aggregate against Unitil (or any of its subsidiaries), (vii) certain defaults in Unitil’s (or its subsidiaries’) obligations under the Employee Retirement Income Security Act or (viii) bankruptcy of Unitil, any of its subsidiaries that becomes a party to any Credit Facility-related documents, or any of their respective subsidiaries. The occurrence of any such events of default would require the repayment of any outstanding borrowings and the termination of the right to borrow additional funds under the Credit Facility.

The lenders under the Credit Facility and their affiliates have various relationships with Unitil and its subsidiaries involving the provision of depository and other cash management and commercial banking services.

Except with respect to the provisions amended by the Fourth Amendment Agreement, all other terms and conditions of the Credit Facility, as amended, remain in full force and effect.

The foregoing description of the Credit Facility, including the Fourth Amendment Agreement, is qualified in its entirety by reference to the full text of the Credit Facility, which is filed herewith as Exhibits 10.1, 10.2, 10.3, 10.4 and 10.5 to this Current Report on Form 8-K.

The disclosure in Item 1.01 of this Current Report on Form 8-K is incorporated herein by reference.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned, hereunto duly authorized.

Date: October 14, 2010

EXHIBIT INDEX