6061-O and 6061-T4 aluminum alloy sheets with 0.04 inch (1.02 mm) nominal thickness are widely used in the aviation industry. After argon arc welding, the sheets were subjected to a solution heat treatment followed by a quenching and tempering treatment (T62 treatment), afterwards fabricated to specimens. The fatigue tests were carried out at 60 Hz, stress ratio of 0.1 both in atmosphere and water at room temperature, and loading direction was perpendicular to welding bead. The fracture morphologies were examined by scanning electron microscopy (SEM). The results show that the fatigue life in water is considerably decreased, and fatigue striation is not so clear compared with that in atmosphere, which might be attributed to the fact that the oxygen content on the fracture surface tested in water after dry treatment is much higher than that in atmosphere.