An improved air compressor includes a cylinder fitted with a piston body, a main frame for mounting a motor, and an air storage container. The cylinder defines a plurality of exit holes regulated by a control mechanism including a resilient sheet having a root and a plurality branches corresponding to the exit holes. The root and branches of the resilient sheet are individually attached to a top wall of the cylinder at separate points, so that each of the branches can be individually moved by the compressed air produced in the cylinder to open or close the corresponding exit hole without affecting movements of the other branches, so that the piston body can conduct reciprocating motion more smoothly, and thus the performance of the air compressor can be increased.