It appears to the authors that the theory of crack opening displacement based on a dislocation model is useful for the analysis of brittle fracture initiation, which necessarily follows the local yield at the tip of the pre-existing crack. The authors carried out Deep Notch Tests with and without welding residual stress and investigated the effect of the residual stress on brittle fracture initiation in steel structure. They also examined the effect of pre-loading on the initiation using same specimens. The test results were analysed using the crack opening displacement theory. Calculation results are in good agreement with the experimental ones. In connection with these results, they discuss the safety of the steel structures with welding residual stresses.