Abstract: A model correlating flow separation with flow loss is proposed for predicting the incipient location and size of the flow separation in a transonic rotor.The flow fields of several rotors have been investigated with the model.The results show that the flow separation mainly occurs in the tip area owing to strong shock boundary interaction,and the incipient separation point is behind the shock location.The tip leakage flow vortex has a strong effect on the interaction between shock wave and boundary layer.