A brushless direct current (BLDC) motor has high reliability and a long life because it eliminates mechanical friction, and compensates for the defects of a direct-current (DC) motor by removing the brush and commutator. Compared to a DC motor, BLDC motor has the excellent efficiency because there is no voltage drop or friction loss of the brush. However, when the motor is operated for a long time with a high output, the internal motor temperature rises, and the copper-winding temperature increases. The temperature rise of the internal motor affects the resistance of the copper winding and the performance of the permanent magnet. This study investigates the efficiency of a 500 W BLDC motor with a varying air flow rate. The temperature rise and efficiency of the BLDC motor are studied by applying inlet flow rates of 0, 30 and 50 L/min during a 20 min load test. The temperature rise is very high at 0 L/min and the efficiencies are calculated as 81.9%, 82% and 82.7%. This study can be used as a basic study on the optimal air flow rate to the internal motor.