Disclosed therein are a heat pump system for a vehicle, which determines that frosting begins on an exterior heat exchanger 130 and carries out a defrosting control if a difference value between outdoor temperature and refrigerant temperature of an outlet side of the exterior heat exchanger is above a frosting decision temperature in a heat pump mode, thereby increasing frost-prevention and defrosting effects and enhancing heating performance and stability of the system because the system recognizes the beginning of frosting on the exterior heat exchanger 130 at a proper time so as to carry out the defrosting control.
