A fan bearing retaining structure includes a base and a ring member. The base has a bearing cup projected from one side thereof, and the bearing cup internally defines an axial bore for receiving a bearing therein. The ring member is fitted around an outer side of the bearing cup and defines an opening, such that an inner side of the opening tightly contacts with the outer side of the bearing cup to apply a radially inward compression toward the axial bore. With these arrangements, a portion of the axial bore corresponding to the ring member fitted around the bearing cup has a reduced inner diameter to thereby effectively retain the bearing in the bearing cup, enabling a fan to be manufactured at reduced cost.