Abstract: The leak of working fluids is inevitably in the actual operation, since the sealing problems in organic Rankine cycle (ORC) such as in components, pipes, etc. Combined Helmholtz free energy model, mixing rule and isothermal leakage model, the variation of mass fraction ratio and cycle performance along with the leakage rate within 0~50% in different positions in the evaporator by using the R245fa/R601a as the working fluid of ORC system were analyzed. The results show that when leakage occurred, the change of the mass fraction ratio of mixed working medium, the thermal properties as well as system performance will take place. With the leakage rate increasing, the specific net work even reduces 12.23%. In addition, the liquid leak in the outlet section of evaporation makes a greatest influence on the system performance.