An air compressor includes a cylinder housing, an outlet receptacle disposed on the cylinder housing, and a partition formed between the cylinder housing and the outlet receptacle and having an air passage formed in the partition, a piston slidably received in the cylinder housing and coupled to a motor for moving relative to the cylinder housing in a reciprocating action in order to generate a pressurized air, and a spring-biased check valve engaged with the partition for selectively blocking the air passage of the partition, the outlet receptacle includes an inner diameter “D2” no less than an inner diameter “D1” of the cylinder housing for allowing the pressurized air to easily flow into the outlet receptacle and for allowing the piston to be moved in a greater moving speed.