The Ono’s rotary bending fatigue test and the cantilever rotary bending fatigue test were carried out on friction-welded 6061 aluminum alloy joints, and the relationship between the deformation heat input in the upset stage or the upset burn-off length and fatigue strength was examined. In the Ono’s type test, sound joints, which fractured in the heat affected zone in the tensile test, fractured in the heat affected zone also and the fatigue limit of these joints was slightly lower than that of 6061 aluminum alloy base metal. This is because joints fractured in the softened area in both tensile test and Ono’s type test using smoothed test specimens. While, in the cantilever type test, the fatigue limit of sound joints was a little more than or a little less than that of 6061 aluminum alloy base metal. It seems that a weld and a structure at the weld interface affected fatigue strength in the cantilever type test using notched test specimens. Judging from the fatigue limit obtained, sound joints could be produced when either the deformation heat input in the upset stage or the upset burn-off length exceeded a certain value.