Ultrasonic vibration has been applied to various molten metal processes owing to the functions of (a) improvement in wettability, (b) liquid adhesion at a vibrating end surface and (c) sono-solidification such as grain refinement. The present study is focused on the sono-solidification with acoustic cavitaion in hypereutectic Al-18mass%Si alloy. There appears an equilibrium microstructure composed of primary silicon and coupled eutectic -Al/Si phases in Al-18mass%Si alloy, however, non-equilibrium -Al grains develop along with the equilibrium phases through the sono-solidification. During the sono-solidification of Al-18mass%Si alloy, non-equilibrium -Al grains are recognized in the molten metal close to the ultrasonic radiator just before reaching the eutectic temperature of 577 oC in addition to the refined primary silicon particles. The appearance of -Al grains is understood through acoustic cavitation: ultrasound in molten Al-Si alloys exhibits two outstanding behaviors of cavitation bubbling and acoustic streaming. Firstly the de-coupled eutectic reaction, which is recognized in the solidified eutectic Al-Si alloy with severe stirring, causes divorced -Al grains by the acoustic streaming with cavitation. Secondly it is expected that high pressure of over 1 GPa generated by the collapse of cavitaion babbles leads to not only an increase in the eutectic temperature, but also higher silicon content at the eutectic point in Al-Si alloy. Consequently, non-equilibrium -Al grains are nucleated at collapsed cavitaion bubble sites, and they are characterized by higher silicon content compared with that of primary -Al grains in hypoeutectic Al-7masst%Si alloy.