EX-99.906CERT

Green Century Funds

Exhibit 12(b) to Form N-CSR

CERTIFICATIONS PURSUANT TO SECTION 906 OF THE

SARBANES-OXLEY ACT OF 2002

I, Kristina A. Curtis, President of the Green Century Funds, certify that to my knowledge:

1. The Form N-CSR of the registrant (the “Report”) fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934; and

2. The information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the registrant.

/s/  Kristina A. Curtis    

Kristina A. Curtis

President and Principal Executive Officer

October 8, 2012

A signed original of this written statement required by Section 906 of the Sarbanes-Oxley Act of 2002 has been provided to the Registrant and will be retained by the Registrant and furnished to the Securities and Exchange Commission (the “Commission”) or its staff upon request.

This certification is being furnished to the Commission solely pursuant to 18 U.S.C. (S) 1350 and is not being filed as part of the Form N-CSR with the Commission.

Green Century Funds

Exhibit 12(b) to Form N-CSR

CERTIFICATIONS PURSUANT TO SECTION 906 OF THE

SARBANES-OXLEY ACT OF 2002

I, Kristina A. Curtis, Treasurer of the Green Century Funds, certify that to my knowledge:

1. The Form N-CSR of the registrant (the “Report”) fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934; and

2. The information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the registrant.

/s/  Kristina A. Curtis    

Kristina A. Curtis

Treasurer and Principal Financial Officer

October 8, 2012

A signed original of this written statement required by Section 906 of the Sarbanes-Oxley Act of 2002 has been provided to the Registrant and will be retained by the Registrant and furnished to the Securities and Exchange Commission (the “Commission”) or its staff upon request.

This certification is being furnished to the Commission solely pursuant to 18 U.S.C. (S) 1350 and is not being filed as part of the Form N-CSR with the Commission.