Exhibit 5.1

August 13, 2010

Atlantic Power Corporation

200 Clarendon St., Floor 25
Boston, Massachusetts 02116

Dear Sirs/Mesdames:

Re: Atlantic Power Corporation

We are acting as Canadian counsel to Atlantic Power Corporation (the “Company”) in connection with the Registration Statement on Form S-1, as amended or supplemented, (the “Registration Statement”) filed by the Company with the Securities and Exchange Commission in connection with the registration under the United States Securities Act of 1933, as amended, in respect of the aggregate principal amount of the Company’s series B convertible unsecured subordinated debentures set out in the Registration Statement (the “Debentures”). The Debentures will be issued under an indenture dated as of December 17, 2009, between the Company and Computershare Trust Company of Canada (the “Trustee”), as debenture trustee, as supplemented by a first supplemental indenture to be entered into between the Company and the Trustee (collectively, the “Indenture”). The Debentures will be convertible into common shares of the Company (the “Common Shares”) on the terms and subject to the conditions set forth in the Indenture, the Debentures and as contemplated by the Registration Statement.

We have examined such records and proceedings of the Company, the originals or copies, certified or otherwise identified to our satisfaction, of certificates of public officials and officers or directors of the Company and such other documents, and have considered such questions of law and made such other investigations, as we have deemed relevant or necessary as a basis for the opinion hereinafter expressed.

In rendering the opinion expressed herein we have assumed:

(a) the genuineness of all signatures, the authenticity of all documents submitted to us as originals, the conformity to authentic original documents of all documents submitted to us as certified, photostatic, notarized or true copies or facsimiles, and the authenticity of the originals of such documents;

(b) the identity and capacity of all individuals acting or purporting to act as public officials; and

(c) that any party to any agreement or instrument referred to herein who is a natural person has the legal capacity to enter into, execute and deliver such agreement or

instrument and has not entered into, executed or delivered the same under duress or as a result of undue influence.

Our opinion is given to you as of the date hereof only and we disclaim any obligation to advise you of any change after the date hereof in or affecting any matter set forth herein.

The opinion hereinafter expressed relates only to the laws of the Province of Ontario and the federal laws of Canada applicable therein and is based upon legislation in effect on the date hereof.

Based upon the foregoing and subject to the qualifications set forth herein, we are of the opinion that:

1. The Indenture has been duly authorized by all necessary corporate action of the Company.

2. The Debentures have been duly authorized by all necessary corporation action of the Company and, upon issuance and delivery in accordance with the terms of the Indenture against payment therefor, will constitute valid and legally binding obligations of the Company, enforceable against the Company in accordance with their terms, subject to the limitations under any applicable bankruptcy, insolvency, fraudulent conveyance or transfer, equitable subordination, reorganization, moratorium or similar laws relating to or affecting creditors’ rights generally and except as enforcement thereof is subject to general principles of equity (regardless of whether enforcement is considered in a proceeding in equity or at law).

3. The Common Shares initially issuable upon conversion of the Debentures have been authorized by all necessary corporate action of the Company and, when issued upon conversion of the Debentures pursuant to the terms and conditions of the Debentures and the Indenture, will be validly issued, fully paid and non-assessable.

We hereby consent to the use of this opinion as an exhibit to the Registration Statement and to the use of our name where it appears in the Registration Statement.

Yours very truly,

/s/ Goodmans LLP

“Goodmans LLP”

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