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A sever deformation layer was fabricated on the surface of AISI 304 austenitic stainless steel by shot peening, which considerably enhanced the efficiency of the following nitriding treatment. The microstructure evolution of austenitic stainless steel during shot peening and nitriding were characterized by optical microscopy and tran mission electron microscopy, and the enhanced diffusion mechanism was discussed. Results indicate that lots of dislocations, slip bands and refined grains are investigated in the sever deformation surface layer after shot peening. Martensite was also observed at the intersection of slip bands, which considered as strain-induced martensite. The strain-induced martensite transformed to supersaturated nitrogen solid solution (expanded austenite) in the following nitriding process. No apparent growth tendency of the refined grains is observed when nitriding temperature is below 450℃. The slip bands and fine grain boundaries induced by shot peening decrease the activation energy of nitrogen diffusion and evidently enhance the nitriding efficiency of austenitic stainless steel.