A new technique was introduced to realize surface nanocrystallization (i.e. generation of a surface layer of nanostructures) on a 316L stainless steel by means of ultrasonic shot peening treatment. The microstructural evolution of the 316L stainless steel was characterized by using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. After the ultrasonic shot peening treatment, obvious grain refinement was observed and a nanocrystalline surface layer was found. The thickness of the nanocrystalline surface layer varies from a few to about 30 ¦Ìm depending upon the treatment duration and the grain size increases from about 10 nm at the top surface layer, gradually to more than 100 nm at a depth of ¡Ö30 ¦Ìm. The nanocrystallization mechanism was analyzed in terms of the deformation behavior and TEM observations of the microstructural evolution of the treated samples.