To investigate the performance improvement of hel icopters by varying rotor diameter, a comprehensive rotor model is combined with a fuselage model. A propulsive trim method is introduced to obtain the pitch cont rols and fuselage attitude in steady states and then the helicopter power requir ed. The helicopter power with rotor radius, forward speed, helicopter weight and fight altitude is investigated to explore how much power reduction can be achie ved. The pitch controls and the tilt of fuselage with rotor radius and forward s peed are also addressed. Varying rotor radius can achieve significant helicopter performance improvement in medium to high forward flight, especially at high sp eed. 20% reduction of rotor radius can reduce the power by 37.6% at the speed 20 0 km/h. The power reduction decreases in low to medium forward flight and in creases at high speed with increasing flight altitude. The collective and cyclic pitches increases with decreasing rotor radius, and the longitudinal and latera l tilt of the fuselage decreases.