The present invention relates to an air conditioner for vehicles, and more particularly, to an air conditioner for vehicles, in which a seat vent is formed on an air-conditioning case to supply some of air existing in a mixing area formed inside the air-conditioning case to the seat and a heat exchanger for seat is also mounted in the air-conditioning case, thereby securing an installation space for installing convenient devices inside a seat, and in which the air conditioner is utilized to cool and heat the seat, thereby reducing consumption of energy by enhancing efficiency, decreasing the number of components since additional temperature-adjusting door is not needed, and supplying a fixed amount of air to the seat regardless of air-conditioning modes.