In　order　to　study　on　tensile　and　fatigue　fracture　mechanism　of　friction　stir　welded　(FSW)　joints,　the　tensile　and　fatigue　behavior　of　FSW　joints　are　studied　based　on　the　microstructure　and　strain　distribution.　The　large　plastic　deformation　and　fracture　occurred　in　the　thermo-mechanically　affected　zone　(TMAZ)　on　retreating　side　in　tension　tests.　High　contents　of　shear　texture　and　small　angle　grain　boundary　reduce　the　tensile　mechanical　property　of　TMAZ　material.　The　fatigue　weak　area　for　FSW　joints　is　affected　by　the　loading　condition.　The　strain　concentration　in　the　welded　nugget　zone　(WNZ)　and　base　material　makes　the　fatigue　fracture　liable　to　happen　in　these　areas　for　the　FSW　joints　under　the　stress　ratios　of　0.1　and　-0.3.　When　the　fracture　occurred　in　WNZ,　the　crack　initiation　mainly　occurred　in　clusters　of　hardened　particles,　while　when　the　fracture　happened　in　base　material,　the　crack　initiation　mainly　occurred　near　the　pit.　The　crack　in　WNZ　propagated　in　an　intergranular　pattern　and　the　crack　in　the　other　areas　extended　in　a　transgranular　mode,　leading　to　a　higher　crack　growth　rate　of　WNZ　than　of　other　regions.