Power bypassed pole sitter Justin Wilson for the lead after a restart and then dueled defending champ Franchitti on the Toronto street course, edging him by 1.28 seconds.
Power took his fourth road course win of the year, including at Watkins Glen, N.Y., two weeks ago. The 29-year-old Australian improved his lead in the IndyCar championship standings, posting a 42-point advantage over Franchitti with Sunday's victory.
Grabbing third place was Ryan Hunter-Reay. Tony Kanaan and Graham Rahal ended in fourth and fifth place respectively while Danica Patrick took the sixth position.
A late-race spin-out relegated Wilson to seventh place.