The effect of ultrasonic treatment on the β-phase (Mg17Al12) morphology of an Mg-9.0wt.%Al alloy was studied. The result shows that with high-intensity ultrasonic vibration employed during the solidifying of Mg-9.0wt.%Al, the β-phase in the entire cross section of the billet is significantly refined and also changed from continuous to discontinuous morphology. Spherical β-phase is formed during the solidification of the billet treated with high-intensity ultrasonic vibration.