Ultrasonic effects on microstructure evolution and mechanical properties of AZ80 alloy were investigated. The results indicated that with the application of ultrasonic melt treatment, the primary α-Mg phases were transformed from coarse dendrites to nearly fine equiaxed grains, and the β-Mg17Al12 phases at α-Mg boundaries were refined and modified into discontinuous. Besides, the tensile properties of AZ80 alloy were apparently improved after ultrasonic melt treatment, in which the 0.2% tensile yield strength, ultimate tensile strength and elongation were improved from 87MPa, 118MPa and 2.1% to 107MPa, 170MPa and 5.4%, respectively.