Abstract An analytical model for predicting the performance of an organic Rankine cycle (ORC) system was reformulated. To do this, a constraint for operating temperature was newly proposed. Typically, based on the Carnot cycle, lots of performance equations for a power system have been derived. Besides, an adequate constraint for the operating temperature has been rarely considered in the previous studies. Those simplicities can increase the chance of error in predicting the performance properly. In this study, the analytical model is comprised of thermodynamic principles and several physical characteristics in the ORC system. Thereby, an analytical expression for the optimal condensation temperature was derived as a function of given parameters. The theoretical model was validated by comparing it with a numerical simulation. Results indicate that the predicted data from the theoretical model are in good agreement with the simulation data. The proposed theoretical model is applicable in the conceptual design of the ORC system or estimating the potential of thermal energy sources.