In the welded structures such as ship and bridge etc. the fatigue cracks which initiate from the parts of stress concentration due to structural discontinuity can be an origin of brittle fracture under some conditions. In this paper, the theoretical and experimental analysis for the brittle fracture initiation characteristics from the cracks which are located along the toe of fillet weld of stiffener to main plate has been made by using the three-dimensional structural models. The following conclusions are obtained from the results : (1) The models with low cycle fatigue crack showed higher toughness than that of the model with machined notch in case of the brittle fracture at low stress level. (2) The brittle fracture stress for the structure with longitudinal stiffener containing a crack along the toe of fillet weld, can be evaluated by taking account of the stress concentration due to the stiffener. (3) Effect of the plastic constraints caused by transverse or longitudinal continuous stiffener on the brittle fracture initiation characteristics after general yielding can hardly been found for the model with a crack at the fillet welds. However, the effect can be found only for the model with a notch at the root of discontinuous stiffeners.