Abstract: This invention relates to a mini-type air compressor and in particular to one which includes a motor having a driving axle connected with a driving gear, a fixed seat on which is fixedly mounted the motor, a transmission gear having an enlarged threaded portion and meshed with the driving gear of the motor, a cylinder mounted on the fixed seat, an eccentric member provided with a shaft extending through the transmission gear to engage with the fixed seat so that one side of the eccentric member is spaced apart from the enlarged threaded portion of the transmission gear and the transmission gear must rotate through a certain distance before that side contacts and pushes the eccentric member to rotate, and a spring mounted between the transmission gear and the eccentric member for pushing the eccentric gear to contact another side of the enlarged threaded portion of the transmission gear.