Phase morphology is one of the most important microstructural characteristics determining the mechanical and therefore the service performance of polycrystalline materials. Now, phase morphology of alloy can be changed by chemical modification, superheating treatment and electromagnetic vibration. The effect of ultrasonic treatment on the Mg17Al12 morphology in Mg-9.0wt% A1 alloy is studied in this paper. The result shows that with employing high-intensity ultrasonic vibration during the solidifying of Mg-9.0wt% Al, the Mg17Al12 in the entire cross section of the billet is significantly refined and its morphology exhibited the change from continuous to discontinuous morphology. Spherical Mg17Al12 is formed during solidification of billet treated with high-intensity ultrasonic vibration.