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A nanocrystalline surface layer was fabricated on a pure Fe plate by using an ultrasonic shot peening (USSP) technique. The USSP induced surface nanocrystallization products were characterized by means of X-ray diffraction (XRD) and transmission electron microscope (TEM). Experimental evidences showed that after USSP treatments, the initial coarse-grained structure in the surface layer was refined into equiaxed ultrafine grains (about 10nm) with random crystallographic orientations. The grain refinement mechanism during USSP treatments is discussed.