With attention to the drawback of specifying isentropic efficiency of expander (or turbine) in Organic Rankine Cycle (ORC) analysis, in order to enhance reliability of analysis results, this article replaces the constant isentropic efficiency by internal efficiency of optimal radial flow turbine for each condition. With both analysis methods, namely internal efficiency analysis method and conventional analysis method, 14 subcritical ORC working fluids are studied with hot water of 90℃, pinch point temperature of 5℃ and condensing temperature of 30℃. Results with both analysis methods are compared. The results show that turbine internal efficiency is determined by expansion ratio in rotor and decreases with the rise of expansion ratio in rotor. There are differences between cycle net power output with internal efficiency analysis method and that with conventional analysis method. The differences can change the results in optimizing fluid. It is significant to apply computational optimal efficiency instead of constant isentropic efficiency in ORC analysis.