This article analyzes the effect of electrostatically actuated flexible microvalves on the output performance of electrostatic micropumps. The existence of microvalves affects the pressure rise and volumetric efficiency of micropumps. The effects of valve volume on the pressure rises of pumps with single and double cavities are predicted by integrating energy minimization with the analytical solutions for circular membrane deflection under uniform pressure. Outlet valve resistance is analytically specified as well. The micropumps could achieve their maximum volumetric efficiency in the case of zero back pressure. However, the design of the inlet valve leads to gas trapping in the inlet valve during discharge. In such a situation, the volumetric efficiency of the pumps drops along with the increase of pressure rise. Moreover, the flexibility of valve plates yields a much lower voltage than the cavity actuation for the valve closing and the breakdown voltage of the dielectric layer on the valve body, thereby enhancing the reliability of the microvalves.