Effects of residual stress and mechanical heterogeneity caused due to welding on brittle fracture initiation in welded joints are analytically investigated by using fracture mechanics. The main results obtained are summarized as follows : (1) The brittle fracture strength of welded joints is intensely affected by magnitude of residual stress rather than form of its distribution (See Fig. 6). The fracture strength can be approximately evaluated by the calculation neglecting the redistribution of residual stress in the vicinity of notch with exception of the much lower stress level (See Fig. 9). (2) Effects of mechanical heterogeneity on brittle fracture strength are equivalent in calculations to those of residual stress. The brittle fracture strength is elevated by the hard region surrounding the notch and is lowered by the soft region (See Fig. 12). (3) The transition temperature ( T _i) _1/2, at which fracture strength becomes equal to half the yield stress σ_Y0 at room temperature, is changed by the factors such as residual stress, mechanical heterogeneity and fracture toughness near the tip of the notch. The elevation of the temperature Δ( T _i) _1/2 due to residual stress is predominent at lower residual stress level (See Figs. 10, 14).