Abstract: The welded joint of SS400 steel was treated by high energy shot peening (HESP) technique, the surface layer was characterized by means of the XRD and TEM,and the changes of hardness, residual stress and fatigue strength were examined.Experimental results showed that nanostructured surface layer might form in the surface layer of welded joint after the HESP treatment, of which the crystallographic orientations were nearly random; therefore uniform microstnzcture could be obtained.The microhardness of the nanostructurcd surface layer was found to be much higher than that of the sample before the HESP treaiment,and the welding residual stresses in this layer became entirely compressive.This kind of nanostructured surface layer with compressive stress could significantly increase the fatigue strength of butt-welded joint.