A heat dissipation structure of a heat-dissipating box fan is disclosed. The structure is used for dissipating the heat generated by the motor of the fan unit. The structure has a bearing sleeve that comprises a hub in the form of a generally hollow cylindrical column and a sleeve base. The hub is inserted inside a central through hole of the fan motor, and the sleeve base covers the base of the plastic frame of the fan unit at the end of the frame opposing the other with the motor installed. A number of heat dissipation fins are formed over the lateral peripheral surface of the base of the bearing sleeve. The hub of the bearing sleeve has a hollow column shape of a polygonal peripheral configuration at the end close to the base and a hollow column shape of a cylindrical configuration for the remaining section thereof.