A method of helicopter rotor power transfer coefficient determination was presented. Firstly, the trim calculation was conducted at various forward speeds to determine the main rotor and tail rotor powers. Then the additional power consumed by accessories other than main and tail rotors was identified by flight test. The variation of rotor power transfer coefficient with forward speed was obtained according to the powers in corresponding forward speed. The result was improved since the rotor transfer coefficient was determined based on the powers in flight. In Addition, the method described in this paper can consider the effect of vertical empennage and different tail rotor type such as ducted tail rotor.