An air compressor contains: a body, a cylinder, a motor, and a transmission mechanism. The body includes multiple positioning orifices which are a first positioning orifice and a second positioning orifice. The cylinder is connected on the body and communicates with an air storage holder. The motor is fixed on the body, a small gear is received in the first positioning orifice, and a connection seat of the motor is accommodated in the first orifice. The transmission mechanism actuates the piston to move in the cylinder reciprocately so as to produce compressed airs. The motor further includes multiple fixing portions extending from a head edge of the casing thereof and configured to engage with the body, thus fixing the motor on the body securely without using any screws.