To improve the overall thermal efficiency of the organic Rankine cycle( ORC), a simulation study was carried out for a combined heat and power( CHP) system, using the Redlich-Kuang-Soave( RKS) equation of state. In the system,R245 fa was selected as the working fluid. A scroll expander was modeled with empirical isentropic expansion efficiency.Plate heat exchangers were selected as the evaporator and the condenser, and detailed heat transfer models were programmed for both one-phase and two-phase regions. Simulations were carried out at seven different heat source temperatures( 80,90, 100, 110, 120, 130, 140 ℃) in combination with eight different heat sink temperatures( 20, 25, 30, 35, 40, 45, 50,55 ℃). Results showthat in the ORC without an internal heat exchanger( IHE), the optimum cycle efficiencies are in the range of 7. 0% to 7. 3% when the temperature differences between the heat source and heat sink are in the range of 70 to90 ℃. Simulations on CHP reveal that domestic hot water can be produced when the heat sink inlet temperature is higher than40 ℃, and the corresponding exergy efficiency and overall thermal efficiency are 29% to 56% and 87% to 90% higher than those in the non-CHP ORC, respectively. It is found that the IHE has little effect on the improvement of work output and efficiencies for the CHP ORC.