An air compressor device includes a cylinder housing and a supporting plate, a piston engaged in the cylinder housing and moved by a motor, the cylinder housing includes a partition for separating the cylinder housing into an upper compartment and a lower chamber, and a passage formed in the partition for allowing the pressurized air to flow from the chamber into the compartment of the cylinder housing, the compartment of the cylinder housing includes an inner diameter (D) greater than an inner diameter (d) of the chamber of the cylinder housing for allowing the pressurized air in the chamber to easily flow into the compartment of the cylinder housing with a relatively decreased pressure.