Ultrasonic vibration was introduced into the molten AZ91 alloy in this paper. The effects of ultrasound power (from 0 to 900W) on the microstructure and mechanical properties of AZ91 alloy were investigated. The results show that the microstructure, hardness and mechanical properties of the AZ91 alloy have been improved with the ultrasonic treatment. In this work, the mechanical properties of the alloy reached the peak value when the ultrasound power was 600W. Also, the mechanism of ultrasonic vibration in the AZ91 alloy was discussed.