Abstract: Magnetorheological (MR) dampers are promising semi-active control devices for vibration control of structures. External energy required by the adjustable fluid damper is minuscule while its response time is on the order of milliseconds. The MR damper is used as the lag damper of rotor and damper of landing gear to suppress adaptively the helicopter ground resonance. Due to the nonlinear properties of MR damper, an equivalent linearized damping coefficient which is more convenient for the analysis of the application of the MR damper in vibration control, is calculated using energy method. Through modeling the coupled rotor/fuselage system of helicopter, the stability area of helicopter ground resonance is obtained. Numerical results indicate that MR dampers can supply the required damping to suppress the helicopter ground resonance under different operating conditions.