In the welded joint, mechanical properties of weld metal and heat-affected zone are different from those of base metal. Brittle fracture initiation behaviors of the weldments will be different from those of the homogeneous and isotropic material. In the present paper, attention in focussed on brittle fracture initiation behaviors of the weldments having heterogeneity under tension parallel to the weld line. Deep notch tests of model specimens (Fig.1) having mechanical heterogeneity near crack tip were carried out. Effects of mechanical heterogeneity on deformation behaviors of crack tip and brittle fracture strength were clarified. Expending the Dugdal's idea, the relationship between the applied stress and crack opening displacement of notched specimens having mechanical heterogeneity can be calculated, and those calculated results agree well with the experimental values. The estimation of fracture stress for this heterogeneous material can be made by COD-criterion.