Small　fatigue　crack　initiation　and　growth　behavior　for　7075　aluminum　alloy　friction　stir　welded　joints　are　investigated　with　the　replica　technique.　The　effects　of　microstructure　on　the　initiation　and　propagation　of　small　cracks　are　studied　under　different　loading　stress　levels.　The　test　results　show　that　the　initiation　and　propagation　of　small　cracks　are　mainly　affected　by　strengthening　particles,　grain　boundaries,　rough　pits　and　hardness　distribution.　Small　cracks　begin　to　grow　when　the　cycle　numbers　are　about　less　than　20%　of　the　fatigue　life　at　the　stress　ratio　of　0.1.　With　the　decrease　of　stress　ratio　to　-　0.3,　the　small　cracks　grow　until　the　cycle　numbers　are　about　30-45%　of　the　fatigue　life.　The　phase　of　small　crack　initiation　and　propagation　accounts　for　about　50-80%　of　the　fatigue　life.　The　growth　rate　of　the　major　crack　is　similar　to　that　of　other　small　cracks　in　the　stage　of　small　cracks　growth.　In　the　stage　of　long　cracks　growth,　the　growth　rate　of　the　major　crack　is　far　greater　than　that　of　other　small　cracks.　The　microstructures　of　different　regions　of　the　joint　have　an　effect　on　the　fracture　mode.