Abstract: Using experimental data of the heavy-duty diesel engine, the system exergy analysis was carried out for a vehicle waste heat recovery system which was an organic rankine cycle (ORC) with R123 and R245fa as working fluids. Results show that under different engine speeds, the R123 system exergy efficiency increases with the increase of evaporation pressure and engine load, but the increase rate decreases gradually. Under different engine speeds, the R245fa system exergy efficiency increases first and then decreases with the increase of evaporation pressure, and increases with the increase of engine load. An evaporator is the key point to improving the system exergy efficiency. An appropriate evaporator for the system can improve the system exergy efficiency effectively.