Abstract: Surface nanocrystallization of 316L stainless steel was successfully realized by means of surface mechanical attrition treatment (SMAT). The microstructural evolution was examined through XRD and TEM, as well as its grain refinement behavior and hardness variation. The experimental results showed that high density dislocations form about 100 μm deep under sample surface, and then the mechanical twins are induced about 50 μm deep with increasing strain value and strain rate. The lamellar twins intercross each other to refine grain. Finally, a nanostructured surface layer about 20 μm thick forms after SMAT process, of which the microstructure consists of a mixture of both martensite and austenite phases with grain size about 10-30 nm. The realization of surface nanocrystallization is considered as resulting from the grain refinement due to dislocation-twin and twin-twin interactions, and its hardness is improved greatly after SMAT.