A series of slow notch bend tests was performed to investigate the effect of residual welding stress on brittle fracture of steel. Chemical composition and mechanical properties of steel tested are shown in Table 1. Following 9 types of test specimens were prepared. (Fig. 1). (1) parent metal, as rolled, notched. (2) parent metal, prebent at room temperature as shown in Fig. 2a, notch side tension, permanent deflection 2. 5mm. (3) parent metal, prebent as shown in Fig. 2b, notch side compression, permanent deflection 2. 5mm. (4) welded specimen, as welded, notched. (5) welded specimen, similar to (2). (6) welded specimen, similar to (3). (7) welded specimen, pretensioned at elastic nominal stress of 25 kg/mm², then notched. (8) do. pretensioned up to yield point. (9) do. pretensioned, permanent strain 2. 5% for 100mm gauge length. Fracture test procedure is the same as Van der Veen test. Temperatures were varied from 0° to -70°C. Fracture appearance are shown in Figs. 3-5. Depth of fibre are shown in Figs. 6-8, and fracture appearance transition temperatures Till are listed in Table 3. Among welded specimens, type (4) (as welded) has highest ( T _II = -18°C) and type (8) (pretensioned up to yield point) has lowest ( T _II=-38°C) transition temperature. Residual stress measurement was performed by resistance wire strain gauge. From the results of our experiment, we can conclude that residual welding stress has a pretty effect on brittle fracture of welded structure.