An ISG system may include a shift stage sensing unit that senses a shift stage of a transmission, a parking sensing unit that senses a driver's intension of parking by recognizing at least one of whether a vehicle enters a parking lot and whether a parking assistant system may be operated, and an ISG controller that stops an idle-stop of an engine when the shift stage of the shift stage sensing unit may be at a reverse stage or the parking sensing unit senses the driver's intension of parking.