A diesel engine exhaust heat recovery system was designed based on organic rankine cycle. Using the thermodynamics model the performance of the system was predicted, and then a diesel engine with and without the exhaust heat recovery system was experimented to validate the performance of the model. The ORC net output power and thermal efficiency were estimated under variable conditions or different working fluid mass flow rates by using the model. The results showed that the net output increased with increasing engine speed. The maximum thermal efficiencies were the same as 12.1% at the different engine speeds, and the best working fluid mass flow rates range for the exhaust heat recovery system was obtained.