The　fatigue　experiment　was　executed　for　the　2219-T6　aluminum　alloy　friction　stir-welded　joints　at　the　rotation　speed　of　800　r/min　and　the　welding　velocity　of　150　mm/min.　Most　fatigue　failures　occurred　in　the　weld　nugget　zone　(WNZ),　the　thermo-mechanical　affected　zone　and　the　nearby　areas.　The　experimental　results　demonstrated　that　the　sudden　hardness　gradient　increases　sites　corresponding　to　the　fatigue　failure　locations.　The　high-angle　grain　boundaries　with　the　highest　concentration　were　scattered　within　the　WNZ.　The　microcracks　initiated　at　the　intersection　of　the　soft　grains.　More　than　one　crack　initiation　site　was　observed　within　the　WNZ　and　the　thermo-mechanical　affected　zone,　when　the　fracture　occurred　in　these　areas.　The　rough　surface　of　the　welding　area　should　be　one　of　the　main　reasons　for　the　fatigue　failure　occurrence.　The　fatigue　crack　growth　rate　in　the　WNZ　at　the　first　stage　was　fastest　in　comparison　with　the　fatigue　crack　growth　rate　in　the　other　areas　of　the　joint.