It is necessary to investigate performance assessment of controlled Organic Rankine Cycle (ORC) systems because the characteristics of controlled process are coupling, time-varying, stochastic disturbances and nonlinearities. In particular, stochastic disturbances occurring in ORC systems are not necessarily Gaussian. A novel control performance assessment (CPA) method based on the entropy of controlled output is developed to evaluate the ORC system controlled by PI and Modeling Predictive Control (MPC). The simulation results indicate that the proposed method is effective and meaningful in the process of actual operation.