In recent years, more and more attention has been paid to organic Rankine cycle (ORC), which is simply structured, highly reliable and easily maintainable. In order to improve the efficiency of ORC systems, zeotropic mixtures whose phase change process is variable temperature, are used as working fluids to match the temperature profiles of the heat source and heat sink. In this paper, a thermodynamic model which mainly includes Jacob number and the ratio of evaporation temperature and condensation temperature is proposed to forecast the thermal efficiency, output work and exergy efficiency of ORC system with zeotropic mixture. Furthermore, the proposed model programmed by Mablab 2010a is verified by the theoretical data. Then, for different heat source inlet temperature, using different zeotropic mixture pairs, output work that is objective function is maximized by optimizing the evaporation temperature. The results show that if the other working conditions are fixed, the heat source inlet temperature has a significant influence on the best composition of zeotropic mixtures at the optimal evaporation temperature. With the increase of heat source inlet temperature, there exists a heat source inlet temperature that pure working fluid has better system performance than zeotropic mixture. The extent of ORC system performance improvement has a positive correlation with zeotropic mixture's temperature glide. (C) 2014 Elsevier Ltd. All rights reserved.