A heat pump system in a vehicle connected with a refrigerant line through which refrigerant flows for controlling cooling/heating of a cabin of the vehicle, including a compressor for compressing refrigerant, an outdoor heat exchanger for condensing high temperature and high pressure refrigerant compressed at the compressor, and an expansion valve for expanding the refrigerant compressed thus, may include an indoor heat exchanger arranged between the compressor and the expansion valve connected to one another with the refrigerant line, and having a partitioned inside to form individual flow passages, and a first valve connected to the compressor, the outdoor heat exchanger and the indoor heat exchanger with the refrigerant line for changing a direction of a refrigerant flow according to cooling, heating, and dehumidifying modes of the vehicle to control the refrigerant flow along the refrigerant line.