Abstract: The operation mechanism of the Wankel expander in organic Rankine cycle are analyzed theoretically and the performance of the expander is simulated by developed thermodynamics model considering leakage among chambers. With R245fa as the working fluid,several thermodynamic parameters of the expander are discussed in a completed working cycle,and the structure parameters of chamber profile including shape factor,rotor profile,intake angle,eccentric distance and wheelbase coefficient are discussed as well. The results show that the Wankel expander can achieve a large expansion ratio,and all the discussed factors can take significant impacts on working performance induding mass flow rate, output power, expansion pressure ration, and volumetric efficiency. Therefore,it is necessary to select optimized parameters for satisfiing performance of the expander in future application.