Effects of varied ultrasonic powers and time on microstructure and properties of AZ31 magnesium alloy were investigated by introducing ultrasonic into the melt,and refining mechanism of ultrasonic on grain size in the AZ31 magnesium alloy was described.The results show that grain size in the alloy is obviously refined with the more uniform distribution as a result of comprehensive role of different effects from ultrasonic,leading to the improvement of mechanical properties.Microstructure and mechanical properties of the alloy are closely related to the ultrasonic power and time.With increasing in ultrasonic power and time,grain size is decreased and tensile strength is increased.However,there exists a critical value for ultrasonic power and time.When ultrasonic power and time exceed the critical value,the tensile strength is decreased with the slowly fining tendency of grain size.Desirable grain size and tensile strength can be observed with ultrasonic power of 600W for 100 s.