Abstract: According to the principle of the role of ultrasound on metal plastic forming, the self-developed ultrasonic friction stir welding machine was used to weld LF21 aluminum alloy plates.The microstructures and fracture appearance of aluminum alloys were studied.Experiments show that the ultrasonic energy can inject into the weld joint effectively.It can not only increase the intensity of the bottom of weld joint, but also improve metal atoms motion and diffusion ability around mixing needle.The grain in microstructure of WNZ(weld nugget zone) is refined and homogenized obviously.The tensile fracture of ultrasonic friction stir welded joint appears to be a dimple pattern.The depth of dimple is shallower and the number of dimples is less than that in the fracture of friction stir welding.The result shows that ultrasonic welding can improve the mechanical properties of the joint, but the elongation decreases a little compared to that of friction stir welding.