In the present study , Harmonic Balance Method (HBM) is applied to investigate the performance of passive vibration isolators with a cubic nonlinear damping characteristic.The results reveal that introducing either cubic nonlinear damping or linear damping could significantly reduce both the displacement transmissibility and the force transmissibility of the isolators over the resonance region.However.at the non-resonance region where frequency is lower than the resonant frequency, both the linear damping and the cubic nonlinear damping have almost no effect on the isolators.