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Ultrasonic effect on microstructure evolution and mechanical properties of Mg-6Zn-0.5Y magnesium alloy was investigated.The results show that ultrasonic treatment can significantly modify the as-cast microstructure of Mg-6Zn-0.5Y alloy.With the application of ultrasonic treatment,the primary α-Mg phase transforms from coarse dendrites to nearly fine equiaxed grains,and the continuous Ⅰ-phase at α-Mg boundaries is modified into discontinuous one.Interestingly,the area fraction of Ⅰ-phase decreases after ultrasonic treatment.Besides,the tensile properties of Mg-6Zn-0.5Y alloy are apparently improved after ultrasonic treatment.Compared to those of the untreated alloy,the ultimate tensile strength and elongation of Mg-6Zn-0.5Y alloy are improved by 67 ％ and 56 ％,respectively.