This paper proposed a thermal efficiency model theoretically based on an ideal ORC to analyze the influence of working fluid properties on the thermal efficiency, the optimal operation condition and exergy destruction for various heat source temperatures were also evaluated utilizing pinch point analysis and exergy analysis. The proposed model exhibits excellent agreements with the theoretical data and shows better performance than the existing models. It also indicates that Jacob number and the ratio of evaporating temperature and condensing temperature have mainly influence on the thermal efficiency of ORC and low Jacob number shows attractive performance for a given operation condition. It is unadvisable to always pursuit of high thermal efficiency for low grade waste heat. According to the evaluation of optimal operation condition, different working fluids have little impact on the optimal operation condition of ORC and selection of working fluid reasonably based on heat source temperature will help to optimize the ORC performance. Working fluid with low critical temperature, low specific liquid heat and high vaporization latent is particularly well adapted for utilization in the ORC. Exergy analysis indicates that the evaporator contributes the major exergy destruction while the condenser has the smallest except the pump. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.