A structure of fixing a bearing of an air compressor, the air compressor contains: a base, a cylinder, and a transmission mechanism. The base includes multiple positioning orifices, and one of the multiple positioning orifices accommodates a bearing. The cylinder is integrally connected with the base and includes an air storage chamber. The transmission mechanism actuates a piston to move in the cylinder reciprocatingly, thus producing compressed air. The first ring of the bearing is non-circular, and a shape of the one positioning orifice of the base corresponds to the first ring so as to avoid an idle rotation of the bearing in the one positioning orifice of the base or a removal of the bearing from the second positioning orifice.