An air conditioner comprises: a first compression unit (C1) and a second compression unit (C2) for respectively compressing a refrigerant; an outdoor heat exchanger (200) provided at an outdoor unit and connected via the secondcontrol valve (400) to the first compression unit (C1) and the second compression unit (C2); and indoor heat exchanger (100) provided at an indoor unit is connected via the secondcontrol valve (400) to the first compression unit (C1) and the second compression unit (C2) and via the expansion valve (700) to the outdoor heat exchanger; and a first control valve (500) for controlling a refrigerant flow by selectively connecting the first compression unit (C1) and the second compression unit (C2) in series or in parallel. As the first compression unit (C1) and the second compression unit (C2) are selectively connected to each other in series or in parallel, a capacity of the air conditioner is varied according to a change of an indoor temperature and fabrication cost is minimized.