In this study, a tandem heat pump was simulated by TRNSYS program. Prior to the simulation, the heat pump was tested at the standard performance conditions according to the KS B ISO 13256-2, which provides that the entering load and source temperatures should be 40℃ and 20, respectively. The heat pump has a rated capacity of 3 RT and an HFC-134a is used as a working fluid. Simulation results show that the heating COP of the tandem heat pump is about 7% higher than that of a conventional single-unit heat pump.