Abstract: Fatigue test was carried out on refilled friction stir spot welding joints of 6061-T6 aluminum alloy to identify the reason for fatigue failure,and the S-N curve and conditional fatigue limit were obtained at test load of 1.5 kN. The microstructure and fatigue fracture were examined by scanning electron microscope (SEM),and the fatigue mechanism and fatigue fracture features were investigated. The results show that the main reason for fatigue failure of refilled friction stir spot welding of 6061-T6 aluminum alloy was the hook in the joint and serious stress concentration at notch tip of the boundary between the upper and lower sheets. The fatigue crack initiated at the edge of the hook, that is,at the notch tip of the boundary between the upper and lower sheets. During welding,optimizing the welding parameters could decrease the configuration of hook,reduce the serious stress concentration at the notch tip,and then improve the fatigue life of refilled friction stir spot welding of 6061-T6 aluminum alloy.