Circular micro plates are used in the many Microelectromechanical devices as micropumps and micro pressure sensors. All such systems exhibit a static instability phenomenon (Divergence) which is known as the ''pull-in'' instability. In this paper a distributed model was used to investigate the pull-in instability of a circular micro plate subjected to non-uniform electrostatic pressure and uniform hydrostatic pressure. The non-linear governing equation was derived and in order to linearize the obtained governing equations, step by step linearization method was used, then the linear system of equation was solved by finite difference method. The obtained results for only electrostatic actuation were compared with the existing results and good agreement has been achieved. There are exist two method of actuation. The pull-in voltages for these two actuation mechanism were investigated and the obtained results exhibited different effects on each actuation mechanism.