Recent progress in fracture mechanics suggests the possibility of its practical application to the purpose of preventing the brittle fracture of steel structures constructed of ductile material such as ordinary structural steels. One of the problems in determining permissible defect size, allowable stress level and service temperature on the basis of laboratory tests is the fact that fracture toughness values such as K_c or Φ_c are not material constants in a strict sense but rather variables depending on the intensity of mechanical constraint of the defective region of the structure. When the deformation at the crack tip in plate thickness direction is highly constrained, multiaxial state of stress is developed, which is supposed to cause the decrease of fracture toughness and the increase of transition temperature. This paper describes the results of an investigation on the brittle fracture initiation characteristics of severely constrained structural member using model specimen with a defect coupled with stress concentration, mechanical constraint and welding residual stress. These three factors are considered to increase the susceptibility to brittle fracture initiation. Comparative experiments have been carried out using wide plate notched tension test (without three factors) and that with longitudinal welds (with residual stress) to clarify the effect of the three factors mentioned above. Further the effects of pre-loading and cyclic loading at room temperature were investigated using the constrained model specimens. COD concept was applied as the fracture criterion for the analysis of the test results.