An alternator includes a stator assembly and a rotor assembly. The rotor assembly includes a rotor core having a first core portion and a second core portion. The rotor core is processed by cementation, so that the first core portion has carbon content that gradually decreases from the outer surface toward the central line of the rotor core till reaching a predetermined carbon content. The second core portion has carbon content lower than that of the first core portion. With these arrangements, the alternator can reduce turn-on speed and increase outputs. A method of manufacturing the non-homogenous rotor core of the alternator is also provided.