An air compressor includes a one-piece injection molded main frame including a vertical portion formed with two horizontal holes, an upper opening between the two horizontal holes and a lower circular opening below the upper circular opening, two screws extending through the two horizontal holes to fixedly mount an electric motor on the vertical portion of said main frame, a shaft rotatably fitted in a lower circular opening and kept in place by a retainer ring, a driven gear fixedly mounted on the shaft and meshed with the driving gear, a flywheel formed with an eccentric projection fixedly mounted on the shaft and located adjacent to the driven gear, an upper part of the vertical portion being formed with a cylinder in which is fitted a piston, a first and second outlet pipes formed on the top of the cylinder, and a cover engageable with a center hole of the cap and the nozzle, whereby the piston can move smoothly within the cylinder and the two outlet pipes can be adapted for use with different nozzles.