Heat transfer enhancement by using ultrasound is treated as a potentially useful technology because it can be electrically controlled and realized with nondestructive method. The purpose of the present paper is to make clear the enhancement mechanism of natural convective heat transfer in the ultrasonic field in terms of cavitation. An experimental system set-up was established to fulfil this purpose. It is found that with the irradiation of the ultrasound, the heat transfer coefficient is increased to some different extent. The maxim increase ratio reaches 3 times at the loop point of the standing wave where cavitation bubbles are easily generated.