A portable air compressor includes a box and a compressor unit accommodated in the box. The compressor unit includes a main frame, a cylinder fitted with a piston body, a motor, and a transmission mechanism. The motor drives the transmission mechanism to have the piston body conduct reciprocating motion in the cylinder to produce compressed air, which is transferred to an air storage container. The cylinder and the main frame are integrally formed of plastic. The cylinder defines an exit hole communicating with an inner space thereof. A metal seat is integrally formed at the cylinder. The central hole of the metal seat communicates with the exit hole of the cylinder. A plug is urged by a compression spring to seal the central hole of the metal seat. The metal seat can endure high temperature within the cylinder to ensure air-tightness between the plug and the metal seat.