Fatigue strength of Al5083-O FSW joints was evaluated in the relation of the FSW conditions. Static strength of the joints in a range of FSW conditions was the same as that of base metal. Because the fracture of the joints occurred at the base metal in the tensile tests. Therefore the joint efficiency in tensile strength of joints with the FSW condition of tool rotation: 800 rpmwelding speed: 100 mm/min (800-100), 800-200 and 500-100 are all hundred percent. However, the fatigue strength of those joints with fine static strength varied very much. The joint efficiency for fatigue strength varies from 75% for 500-100 to 31% for 800-200. Crack path in fatigue test was always initiated at the center of back surface of FSW weld zone, and propagated through stir zone. Crack growth rate measurement and fracture surface analysis resulted the difference of the joint efficiency has a connection with the incomplete welding at back surface of joints.