The present authors have investigated on brittle fracture initiation in welded structures, using 600mm wide, transversely butt-welded plates of 80 kg/mm² class high tensile steels. They found that brittle cracks initiated at fusion zone of one surface, and immediately propagated through the heat-affected zone. Fracture stresses, in some cases, were lower than yield stress of the base metal. Such brittle fracture, occuring along the brittle zone of welded joint at low stress level, was considered to be a characteristic phenomenon to welded structures; it could hardly be reproduced. in any of the existing testing methods.