A method of mounting a bearing to an air compressor including a shaft element having a first end defining a central cylindrical recess and a second is disclosed, which includes the steps of: fixing the second end of the shaft element to a center of a gear; inserting the first end of the shaft element through a central hole of a bearing to have an annular step of the shaft element abutted an inner ring of the bearing; and hitting the first end of the shaft element by a striking tool with a central pin to form an outwardly extending rim on the first end of the shaft element. With the method, the bearing can be firmly fixed between the outwardly extending rim and the annular step of the shaft element.