An air compressor (10) is received in an accommodation box (8) and contains a body (1), a cylinder (2), a motor (4), and a transmission mechanism (6). The body (10) includes a first positioning orifice (11) and a second positioning orifice (12). The cylinder (2) is connected on the body (1) and communicates with an air storage holder (3). The motor (4) is fixed on the body (1), a small gear (61) is received in the first positioning orifice (11), and a connection seat (41) is accommodated in the first orifice (11). The transmission mechanism (6) actuates a piston (5) to move in the cylinder (2) reciprocately so as to produce compressed airs. The motor (4) includes at least one locking extension (46) for engaging the motor (4) with the body (1). A first end of a respective one locking extension (46) extends from the motor (4), and a second end of the respective one locking extension (46) is engaged on the body (1), hence the motor (4) is fixed on the body (1) securely without using any screws.