The preparation of semi-solid slurry of hypereutectic Al-Si alloy by Ultrasonic Vibration (USV) has been studied. The A390 alloy melt was poured into a preheated metal cup, and subsequently the ultrasonic vibrator was dipped into the melt and USV was imposed. The solidified microstructure of the slurry has been analyzed, and the mechanical properties of the alloy have been tested by making samples through diecasting of the slurry. The results show that the primary Si particles of A390 alloy became finer, rounder and uniformly distributed in microstructure of A390 alloy slurry if imposed with USV, and average diameter of primary Si particles was about 20μm. With USV, the ultimate tensile strength and the hardness of rheo-casted samples increased by over 25% and near 50% respectively besides the elongation rose by above 100%. After heat treatment, the mechanical properties were further improved. In addition, the mechanism of preparation of semi-solid slurry of hypereutectic Al-Si alloy by USV is discussed.