This paper presents a simulation model based on an actual Organic Rankine Cycle (ORC) bottoming system of a diesel engine. The ORC system is built to recover waste heat both from engine exhaust gas and jacket water using R245fa as working fluid. Simulations and thermodynamic analyses are conducted to observe the influence of evaporating pressure and diesel engine (DE) conditions on system performance. Comprehensive evaluations are carried out on waste heat absorbing, expansion power, system efficiency, exergy loss and exergy efficiency. The combined system of diesel engine with bottoming ORC (DE-ORC) is finally investigated. Results indicate that, approximately 75% and 9.5% of waste heat from exhaust gas and from jacket water respectively can be recovered under the engine conditions ranging from high load to low load. The ORC system performances well under the rated engine condition with expansion power up to 14.5 kW, recovery efficiency up to 9.2% and exergy efficiency up to 21.7%. Combined with bottoming ORC system, thermal efficiency of diesel engine can be improved up to 6.1%. (C) 2012 Elsevier Ltd. All rights reserved.