To predict performance of a refrigerating cycle it is effective to use a computer simulation program which employs mathematical models of the cycle components. This paper describes a mathematical model of a rotary compressor which can expersses transient behavior of the compressor with variable operting speed. The compressor is divided into two control volumes and two control bodies, and the model is expressed by six governing differential equations on specific enthalpies of refrigerant in the control volumes, mass flow rates of refrigerant delivered from the volumes and temperatures of the bodies. Simulation results and experimental data are favorably compared for transient behaviors of the compressor under wide operating speed range. The model developed will be effectively integrated into cycle simulation of dynamic responses of inverter driven heat pumps.