Surface　mechanical　attrition　treatment　(SMAT)　was　performed　on　a　reduced　ferritic/martensitic　(RAFM)　steel　to　form　a　nanostructured　(NS)　layer　on　the　surface　of　the　sample.　Both　electron　backscatter　diffraction　(EBSD)　and　TEM　were　used　to　investigate　the　microstructure　evolution　during　SMAT.　The　experimental　results　showed　that　there　were　three　different　zones　after　SMAT:　(i)　The　＂ultrafine　grain＂　(UFG)　zone,　observed　at　the　top-most　surface　region,　(ii)　the　＂transition　zone＂　in　which　the　original　grains　were　fragmented　under　the　severe　plastic　deformation　and　(iii)　the　＂deformed　zone＂　in　which　the　original　grains　were　simply　deformed.　The　average　grain　sizes　increased　rapidly　with　the　increase　of　depth,　while　the　Vickers　hardness　decreased　with　the　increase　of　depth,　and　this　phenomenon　could　be　explained　in　terms　of　boundary　strengthening　and　dislocation　strengthening.　The　number　fractions　of　medium-angle　grain　boundaries　(MAGBs)　and　medium-high-angle　grain　boundaries　(MHAGBs)　in　UFG　zones　were　larger　than　those　in　the　transition　zone　and　the　deformed　zone.　However,　the　number　fraction　of　the　low-angle　grain　boundaries　(LAGBs)　was　extremely　small　in　all　the　zones　after　SMAT,　especially　in　the　transition　zone.