The amount of low temperature heat resources is very huge, efficient utilization that energy is very important issue for improving energy efficiency, saving energy and protecting environment. Due to the small available energy of low temperature heat source, how to improve thermodynamic efficiency is the key problem. In this paper, the thermodynamic model of low temperature thermal power conversion system based on organic Rankine cycle was described firstly. Turbine, single screw and piston expanders were briefly described. R123, R245fa and R134a were chose as working fluid because of quite different critical temperature. Based on this model, the influence of thermodynamic property of organic working fluid on the efficiency of low temperature thermal power conversion system was discussed. The calculating result showed that R123 is the best choice if no considering the impact of expander types and that R245fa is the best choice if considering the impact of expander. This conclusion indicated that it is very important to investigate the match relationship between working fluid and expander. Moreover, single screw expander was proved to be more suitable than turbine and piston expanders for low temperature heat power conversion system.