The　hardness　and　microstructure　of　friction　stir　welded　(FSW)　6082　aluminum　alloy　joint　were　investigated　by　Vickers　microhardness　test,　optical　microscopy　(OM),　electron　backscatter　diffraction　(EBSD)　and　transmission　electron　microscopy　(TEM).　The　hardness　distribution　is　in　a　W　shape,　and　from　the　base　metal　to　the　heat　affected　zone　(HAZ)　the　hardness　decreases　from　103　HV　to　72　HV,　then　gradually　increases　to　84　HV　at　the　nugget　zone　(NZ).　The　grains　of　base　metal　(BM)　are　elongated　and　composed　of　a　great　quantity　of　low-angle　grain　boundaries.　The　nugget　zon　was　of　quite　fine　recrystallized　grains.　For　the　thermomechanical　affected　zone　(TMAZ),　the　grain　size　is　a　little　smaller　than　that　of　base　metal　and　some　low-angle　grain　boundaries　remain.　In　the　heat　affected　zone,　the　grain　size　was　similar　to　that　of　the　base　metal.　The　β　phase　(Mg5Si6)　and　Al-Mn-Si　particles　are　dispersed　in　the　base　metal.In　the　heat　affected　zone,　β　phase　transforms　to　β　phase　(Mg9Si5).　The　hardness　distribution　in　a　W-shape　was　discussed　on　the　basis　of　grain　size,　density　of　low-angle　grain　boundary　and　secondary　phases.　©　2020　Trans　Tech　Publications　Ltd,　Switzerland.