Abstract: A miniature air compressor includes a motor having a shaft for driving a crank. A cylinder has an open end allowing a second end of the crank extending into the cylinder for compressing the air and a close end opposite to the open end of the cylinder. The cylinder has an inlet and an outlet respectively defined in the close end thereof and extending to communicate with an inner periphery of the cylinder. A cover longitudinally mounted to the close end of the cylinder. An input passage and an output passage are respectively defined in the cover. The input passage and the output passage respectively communicate with the inlet and the outlet of the cylinder. A valve sheet secured between close end of the cylinder and the cover. The valve selectively closes the outlet in the cylinder and the input passage in the cover.

Abstract: A miniature air compressor includes a motor, a compression cylinder, a lower valve seat, an upper valve seat, and a noise suppression cylinder. Thus, the miniature air compressor may provide a stable pressurized air flow whose noise and impurity may be eliminated by the noise suppression cylinder, thereby enhancing the usage effect of the injection pen device. In addition, the miniature air compressor has a simple construction, may be assembled and amended easily and conveniently, and may reduce the cost of production.