A theoretical formula is proposed to calculate the OET (optimal evaporation temperature) of subcritical ORC (organic Rankine cycle) based on thermodynamic theory when the net power output is selected as the objective function. The OETs of 22 working fluids including wet, isentropic and dry fluids are determined under the given conditions. In order to compare the accuracy of these results, the quadratic approximation method in EES (Engineering Equation Solver) is used to optimize the net power output and the OETs are obtained by numerical simulation. The results show that the OETs calculated by the theoretical formula are consistent with the numerical simulation results. In addition, the average computational accuracy of OETs from the theoretical formula is higher than that from the simplified formula recommended by the related literature. The larger net power output will be produced when the critical temperature of working fluid approaches to the temperature of the waste heat source. According to the maximum net power output, suitable working pressure, total heat transfer capacity and expander SP (size parameter), R114, R245fa, R123, R601a, n-pentane, R141b and R113 are suited as working fluids for subcritical ORC under the given conditions in this paper. (C) 2011 Elsevier Ltd. All rights reserved.