For the deformation problem of electrostatically actuated microbeams used in MEMS, based on the parallel-plate prototype, pull-in behaviors of a cantilever microbeam have been analyzed by selecting the modal shape functions of cantilever beams as admissible trial function and using Rayleigh-Ritz energy method. Approximate analytical expressions for pull-in voltage and normalized pull-in displacement of a cantilever microbeam at the free end have been obtained. When the pull-in occurs, the pull-in voltage and normalized pull-in displacement of the cantilever beam at the free end are 73.95V and 0.448, respectively. Compared to the results reported in other literatures, it is shown that the obtained analytical solution possesses high accuracy.