An air conditioning system for motor vehicles includes an air conditioner case, a cold air path through which a cold air flows, a warm air path through which a warm air flows, a temperature control door arranged to swing between the cold air path and the warm air path so as to control an opening degree of the cold air path and an opening degree of the warm air path, a defrost vent arranged to discharge the cold air and the warm air toward an upper region of a vehicle room, a bypass duct arranged to bypass a part of the warm air toward the defrost vent, the bypass duct including an inlet aligned with the warm air path and an outlet aligned with the defrost vent, and a cold air introducing portion arranged in at least one of the inlet and the outlet of the bypass duct.