In the soft welded joints having notches in the weld metal, plastic deformation at the notch tip will be held in check by the stronger base metals. In the present report, effect of plastic constraint caused due to mechanical heterogeneity on brittle fracture initiation is investigated. Notched wide plate with groove tensile tests (Groove Deep Notch Test) were carried out as a clue to elucidate effects of plastic constraint on plastic behaviors at crack tip and critical crack opening displacement, and deep notch test of soft welded joints which have saw cut notches in weld metal was carried out. The main results obtained are summarized as follows : (1) As the plastic constraint factor becomes larger, the applied stress required to produce a constant crack opening displacement becomes larger (Fig. 9) and the critical crack opening displacement Φ _c becomes smaller (Fig. 10). The brittle fracture net stress is maximum at a certain value of plastic constraint factor (Figs. 3, 11). (2) The relations between the applied stress and crack opening displacement for various values of plastic constraint factor can be calculated by considering increase of the general yield stress due to plastic constraint (Figs. 9, 16). (3) In the practical soft welded joints, the brittle fracture strength is affected more considerably with fracture toughness than yield strength of weld metal.