One of key targets in helicopter design is to be free from ground-resonance, which may
occur when lead-lag frequency of rotor is coupled with natural frequencies of rotor-fuselage-landing gear system. Typical analytical approach to this problem is based on simplified linear modeling for the landing gear. In practice, however, the landing gear has nonlinear characteristics not only when touching down but also when operating on ground, depending on stroke, velocity and friction of oleo-pneumatic shock absorber in the landing gear. In this study, a method is proposed to predict the vibration severity and unstable range of rotor speed, by including the nonlinear characteristics in the landing gear. Then, effects of landing gear nonlinearities will be discussed.