Recently it was reported that several large welded steel structures fabricated of high strength steels, such as L. P. G. spherical tanks, fractured in a brittle manner under low applied stress levels. The main causes to be thonght in these fractures were the weld defects such as a notch, angular distortion of welded joints and residual stress due to welding. In this paper, the effect of the superposition of the above defects on brittle fracture initiation characteristics of welded joints for 60 kg/mm² high strength steels were investigated by conducting the wide tension test specimens. Although both the steels used for specimens and welded joints had high tensile strength and sufficient notch toughness for practical use in general structures, the specimens initiated brittle fracture at low stress levels below yield point by the superposition of the weld defects. It was found from the experimental and theoretical researches that the superposition of the weld defects had various influences upon the fracture initiation. In addition, it was proved that mechanical stress relieving such as preloading prevented the welded joints of the high strength steels from fracturing at low stress levels, even if the above defects were superposed.