The invention provides a micro-electro-mechanical system pressure sensor. The micro-electro-mechanical system pressure sensor includes: a substrate, including at least one conductive wiring; a membrane disposed above the substrate to form a semi-open chamber between the membrane and the substrate, the semi-open chamber having an opening to receive an external pressure; and a cap, disposed above the membrane and forming an enclosed space with the membrane, the cap including a top electrode corresponding to the membrane and at least one portion of the membrane forming a bottom electrode, wherein the top and bottom electrodes form a sensing capacitor to sense the external pressure.