An external rotor structure used in a motor structure is made of lightweight and low-cost polymer plastic material and provides good heat convection to provide vehicle cooling. The external rotor structure has a ring-shaped yoke iron part, a ring-shaped magnet part and a plastic part. Thus, by using a design of covering the ring-shaped yoke iron part and the ring-shaped magnet part with polymer plastic material by in-mold injection, it achieves the smooth operation and balance of the motor structure. Combined with through holes in the polymer plastic, the heat generated during the operation of the motor structure can be smoothly discharged to the outside, and the temperature difference between the inside and the outside caused by the operation of the motor structure can be balanced by heat convection. In short, the external rotor structure has advantages of lightweight and low cost.