Hunter Pence and Andrew Susac homered, and Travis Ishikawa drove in three runs with pair of doubles among three hits to help the Giants strengthen their hold on an NL wild card spot.
Peavy (2-3) has won two straight after losing his first three starts with San Francisco following a July 26 trade from Boston where he was 1-9.
He allowed 10 hits, struck out eight and walked none. Chris Valaika's two-run homer in the second inning accounted for the only runs against him.
The Cubs' Luis Valbuena added a solo homer and a triple in going 3 for 4.
