Abstract: A nanostructured surface layer was fabricated on a J507 weld metal by. ultrasonic impact peening (UIP). The refined microstructure in the top surface layer was characterized by means of X-ray diffraction and transmission electron microscopy (TEM), and the microhardness variation along the depth of the treated sample was examined. Experimental results show that, after the UIP treatment, the microstructure of the surface layer may be refined into 21.25 nm. Grains refinement involves formation of dense dislocation walls (DDWs) and dislocation tangles (DTs) in coarse grains, transformation of DDWs and DTs into subboundaries, and evolution of subboundaries to highly misoriented grain boundaries. The strengthened thickness of the layer is 100 μm after UIP treatment. The microhardness of nanocrystalline surface layer is enhanced significantly after the UIP treatment compared with that of the original sample.