Disclosed is an air conditioning system for a vehicle which includes a cold air passageway having a heat exchanger for cooling and a warm air passageway having a heat exchanger for heating inside an air-conditioning case to carry out cooling and heating and which controls to inhale indoor air inside the vehicle, pass the air through the heat exchanger for cooling and discharge the air outside the vehicle in a specific mode, thereby preventing frosting of the heat exchanger for cooling because supplying air of high temperature dehumidified inside the interior of the vehicle to the heat exchanger for cooling, and enhancing heating performance by rising refrigerant pressure and temperature in the refrigerant cycle without needing additional components.