By spplying the ultrasomic wave to the solidifing liquid metal, grain sizes of the ingot are refined and the structures are homogenized. One of the reason is the enhancement of nucleation by ultrasonic vibration. According to the reports on this subject, the uncleating conditions are formed by the increse of melting point which is produced by the high pressure due to caviation. In this paper we study whether the caviations are the orgin of the nucleation and analized whether the nucleating conditions are formed by the increase of melting point or not and also compared the analytical result with the experim nt. We analized the pressure change induced by cllaphsing of cavity and the motion of cavith in oscillating pressure field. And we further analized the variation of melting point with pressure change.