In this paper, the fatigue strength of welded joints of 6N01 aluminium alloy extrusions is studied. Less copper content ( C_u _??_0.02%) alloys were chosen, taking corrosion resistance to sea water into consideration. Two series of specimens were prepared. One is non-load carrying transverse fillet welded cruciform joint (T-TYPE), and the other has two longitudinal stiffeners welded on both sides of the main plate (L-TYPE). For L-TYPE, three types of specimens, changed in details of end of longitudinal stiffeners from the conventional corner boxing, were arranged. For some specimens, weld toes were ground with a pencil grinder. Using the specimens, fatigue tests under axial tensile loading ( R =0) were carried out. In the fatigue test results, evident differences existed. The fatigue strength of ground T-TYPE specimen was highest, and that of as welded L-TYPE ones was lowest. The fatigue strength of as welded T-TYPE specimens was higher than that of ground L-TYPE ones. These SN data were consolidated and yielded to one SN curve, using the equivalent stress, taking the maximum stress into account. The equivalent stress S _??_ was expressed as follows; where S _max=maximum stress, R =stress ratio, m =constant, S =stress range, K =stress concentration factor, S_n =nominal stress range, and S_R =residual stress. In the last part of the paper, some techniques for improving the fatigue strength of corner boxing welds are illustrated.