Abstract: Surface treatment was carried out on the weld toe of welded butt joint of MB8 magnesium alloy with HJ-Ⅲ type ultrasonic impact machine. The comparative fatigue test both for treated and un-treated specimens were carried out using EHF-EM200K2-070-1A type electro-hydraulic servo fatigue machine. The microstructure in the top surface layer of weld toe was characterized by means of optical microscope(OM) and transmission electron microscopy(TEM). Experimental results indicate that when the fatigue life is 2×10⁶, the fatigue strength of treated specimen is 52.8 MPa, the strength has been increased about 37.5%. Compared to the un-treated specimen, the fatigue life of treated specimen has been increased 58-65 times. After impact treating, the stress concentration has been decreased, the severe plastic deformation in the surface of weld toe was formed, the thickness of the plastic deformation layer is about 70 μm, and the nano-grains were obtained on the treated surface. The tensile welding residual stresses can be reduced significantly, even change to compressive stresses. The fatigue life of MB8 magnesium alloy can be greatly improved by the ultrasonic impact treatment.