In this paper the formation and growth of bubbles during the melt foaming process are studied from microcosmic aspect. The forming process of liquid film between two neighboring bubbles is also analyzed. According to the cavity effect of ultrasonic wave, the effect of ultrasonic wave on the fragmentation of cell structures of aluminum foam is investigated. Results show that the effect of ultrasonic wave on the cell structure fragmentation decreases with the increase of the distance from the ultrasonic wave source to the cell structure. Therefore, a new concept, namely aluminum foam of biomimetic gradient structure, is proposed.