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The Rare Earth (RE) containing Mg cast alloys have big potential to be used for heat-resistant applications, on condition that the RE-containing intermetallic compounds can be refined during the casting process. In this research, the effect of Ultrasonic Vibration (USV) with different powers on microstructure and mechanical properties of the semi-solid Mg-3RE-3Zn-0.7Y alloys has been investigated. The Mg alloy melt at temperature of 20?40 癈 above the liquidus was exposed to USV for 90 s with different powers from 800W to 1200W, and the USV ended about 10癈 below the liquidus. The results show that good semi-solid slurry with fine and spherical α-Mg particles could be obtained, and the primary α-Mg crystals are with average particle size of 55 μm and average shape coefficient of 0.63 after USV with 1000 W power. After USV treatment with the power of 1000 W, the ultimate tensile strength and elongation of the casting samples were 25.2% and 93.5% higher than those of the samples without USV treatment respectively. It can be concluded that ultrasonic vibration is a good method to prepare semi-solid slurry of Mg alloy with fine and relatively round primary crystals due to cavitation and acoustic streaming effects.