316L　stainless　steel　plate　was　treated　by　means　of　surface　mechanical　attrition　treatment　(SMAT)　technique,　and　a　gradient　structure　consisting　of　gradually　increased　grain　size　from　nano-scale　in　the　surface　layer　to　micro-scale　in　the　matrix　was　obtained.　The　SMAT　sample　was　annealed　in　vacuum　at　different　temperatures,　and　the　structural　evolution　was　analyzed.　The　results　show　that　no　obvious　change　can　be　found　for　the　gradient　structure　when　the　annealing　temperature　is　lower　than　0.5　Tm　(Tm　is　melt　point　of　the　steel).　With　increasing　annealing　temperature,　recovery　and　grain　growth　occur　in　the　gradient　structure,　however　the　grain　growth　rate　is　very　low.　The　release　of　internal　stress　and　micro-strain　induce　mechanical　twinning　below　surface　layer　and　martensite　transformation　in　the　nanostructured　surface　layer　during　annealing.