Ultrasonic vibration is applied to the solidification process of Al-Si alloys, with a horn inserted in the molten and semi-solid alloy. It is found that the primary crystals in the alloys can be refined to different extents depending on the temperature where the ultrasonic vibration is applied. In the Al-Si alloy with a hypo-eutectic composition, the dendrite primary crystals are apparently segmented by applying the ultrasonic vibration in the temperature range from 973 K to 883 K. On the other hand, the segments are found only in the local regions near the horn when the ultrasonic vibration is applied in the semi-solid temperatures, i. e. from 883 K to 850 K. When the primary crystals are angular Si in the Al-Si alloys of hyper-eutectic composition, the vibration can even refine the primary Si grains in the semi-solid area. Modeling the movement of molten alloy in mold with the application of ultrasonic vibration, an acoustic stream is feasibly caused about multiple nucleator in the molten alloy. It is therefore concluded that the primary crystal segments are dispersely distributed all over the molten alloys by such an acoustic stream, which play a role of prompting the nucleation of primary crystals.