In this paper, for the purpose of reproducing brittle fracture at low stress levels in the welded joints of 80kg/mm² high strength steel, we have carried out a systematic investigation on fracture load conditions of surface notched wide tension test specimens, continued from the previous report about 60kg/mm² high strength steel. Further, we considered to obtain a substantiation of fracture conditions, and suggestions for the prevention of similar troubles in future, by application of fracture mechanics for various experimental fracture conditions. The main results obtained are summarized as follows. (1) Brittle fracture in the welded joint of 80kg/mm² high strength steel was reproduced below yield stress levels in practical service temperature regions, by the superposition of various actual stress concentration factors such as a surface notch, angular distortion of the welded joint, welding residual stress, etc. (2) Stress concentration factors are severer, the fracture transition region and stress transition region in wide tension test specimens are closer to each other. (3) By application of Irwin's fracture mechanics it was possible to analyse the experimental results approximately, although the brittle fracture initiation was in a considerably high temperature region. (4) Even when various stress concentration factors are present in superposition, it is possible to prevent the fracture initiation of the welded structure under low loading condition by mechanical stress relieving such as preloading in the higher-temperature region than the service temperature.