In this paper, the energy balance method has been successfully used to study a nonlinear oscillator arising in the microbeam-based microelectromechanical system (MEMS). Firstly, the governing equation of the free vibration of a microbeam is governed based on the Euler-Bernoilli hypothesis where the midplane stretching effect and distributed electrostatic force are both considered. Then this PDE problem is simplified into an ODE problem by using the Galerkin method. Finally, the nonlinear ODE equation is solved by a powerful mathematical tool, the energy balance method. The good agreement of results got from energy balance method with results from fourth-order Runge-Kutta method indicates that the obtained period is of high accuracy. (C) 2010 Elsevier B.V. All rights reserved.