Abstract: A dynamic model for thetime-variable-electric-capacity-type electrostatically actuatedmicro-electro-mechanical systems (MEMS) under parametric excitations ispresented with the effect of squeeze film damping. The harmonic balance (HB)method is applied to simulate the frequency response of this system underthe combined parametric and forcing excitations. The effects of the appliedvoltages and frequency ratios on the frequency responses of the system arediscussed. In addition, the nonlinear dynamic characteristics of this systemare studied with the control parameters of the amplitude of the alteringcurrent voltage, frequency ratio and squeeze film damping ratio. Thetheoretical results indicated the nonlinear dynamic characteristics,bifurcation and chaos of the electrostatically actuated MEMS underparametric excitations. Furthermore, it was also indicated that the effectof squeeze film damping on the dynamic characteristics of the system cannotbe ignored. This investigation provides an understanding of the nonlineardynamic characteristics of electrostatically actuated MEMS under parametricexcitations at micro-scale.