A regenerative organic Rankine cycle (RORC) system is designed to recover the exhaust heat of diesel engine. The organic working fluids under investigation are the pure fluid R245fa and the zeotropic mixture Mix (isopentane and R245fa mixed in 7/3 mole fraction). The influence of intermediate pressure (pressures at which the steam is extracted from the expander) on the performance parameters of the RORC system, such as the net power output, the thermal efficiency, and the mass flow rate of the working fluid, are analyzed, and on the basis of the analysis results, the intermediate pressure is set to 1.15MPa for the RORC system using Mix and 1.2MPa for the RORC system using R245fa. Then the performance of the RORC system using the two different working fluids is analyzed over the whole operating range of the diesel engine, respectively. The calculation results show that Mix is the better selection for the RORC system than R245fa.