A protection structure for outer shell of a hub comprises: a hub and an outer shell mounted at one end of the hub, the outer shell is provided at one end close to the hub with an enlarged flange, on an outer peripheral surface of the outer shell are annularly and spaced-apart formed a plurality of splines and at least one positioning rib, at least at one side of each splines is disposed with a protecting member whose material is different from that of the outer shell. By such arrangements, the protecting member can be assembled with a sprocket to prevent the sprocket from touching the splines. Thus, damages of the outer shell of the hub are prevented.