A cooling system of a AC generator for vehicles has a stator with a stator winding, a rotor that is arranged in an inner circumferential side of the stator, a rectifier that converts an alternating current voltage generated in the stator winding into direct current, and a rear cover that covers the rectifier. The rear cover has an intake duct connecting portion that projects outside and an air intake duct is attached to a tip portion of the intake duct connecting portion. A dividing wall that divides an internal space of the rear cover into a plurality of spaces along a direction extending from the rectifier backwardly relative to the direction of rotation is disposed in the intake duct connecting portion.