In general, the element (Sr, Na, Sb, etc.) is used as a modifier of eutectic Si phase in the hypoeutectic Al-Si system. When these elements are added into the hypoeutectic Al-Si melt, the flake shaped Si phase transforms to fibrous shape and the size of Si phase is also decreased. In this study, the electromagnetic vibration is adopted for modifying eutectic Si phase and reducing its size. The higher the current density and frequency of electromagnetic vibration(EMV), the finer the size of eutectic Si phase. The tensile strength and elongation of EMVed alloy were highly improved. Measured twin probability of EMVed alloy at a frequency of 1000Ffz was approximately six times as high as that of the normal alloy and a half of that of Sr modified alloy. The mechanism for the increase in twin density due to EMV during solidification could be supposed from the fact that the preferential growth along <112> in silicon was suppressed by preventing Si atom from attaching to the growing interface of Si phase and by changing the solid/liquid interfacial energy of silicon.