The system performance analysis and optimization of an organic Rankine cycle (ORC) system using HFC-245fa (1,1,1,3,3-penta-fluoropropane) as working fluid driven by exhaust heat is presented. The thermodynamic performances of an ORC system under disturbances have been analyzed. The results show: maximizing the usage of exhaust heat as much as possible is a good way to improve system output net power and efficiency; the degree of sub-cooling at the condenser outlet should be small (0.5-0.6 K); when the ambient temperature is too high, the system output net power and efficiency will deteriorate with the departure from nominal state possibly exceeding 30%. According to the running environment, choosing a proper nominal state is a good idea for improving the system output net power and efficiency. (c) 2006 Elsevier Ltd. All rights reserved.