The surface of AZ91D magnesium alloy was treated using HJ-III type ultrasonic impact treating (UIT) machine and the grain refining mechanism of ultrasonic impact treating was investigated by means of HTEM. The wear resistance of AZ91D magnesium alloy was experimentally studied both for the treated and un-treated samples respectively. The effect of UIT on the microstructure, and micro-hardness were also investigated using SEM and micro-hardness tester. The experimental results indicate that the severe plastic deformation in the surface of AZ91D was formed by UIT, the thickness of the plastic deformation layer is about 120 祄, and the grains are highly refined and compressive residual stress in the surface of AZ91D was also obtained. The wear resistance and micro-hardness of AZ91D alloy could be greatly improved via UIT. The wear resistance and micro-hardness are promoted by increasing the UIT current and time, which indicates that the UIT is a useful method to increase the hardness and wear resistance of AZ91D alloy.