In this paper, the coupling electromechanical model is proposed for a cantilever microbeam in microelectromechanical system (MEMS). Based on mechanics of plate and theories of electrostatic field, the coupling electromechanical behavior of the cantilever microbeam is investigated. The pull-in voltage is accomplished using the increment-iterative method. The effectiveness of the proposed method is verified by comparing numerical results with that in the literature. The numerical results show that the pull-in voltage decreases with the improvement of the ratio of the width of micro beam to the initial gap between the electrodes.