The CTOD design curve of WES 2805-1997 is widely used for the flaw evaluation from fitness for purpose view points. In the design curve, welding residual stresses are treated as constant magnitude strains or CTODs, therefore residual stress effects on brittle fracture strength are overestimated in low fracture toughness regions. In this study, evaluation methods of effect of previous loads, such as welding residual stress and preload by proof test etc., on the brittle fracture strength are ivestigated and a modification of the CTOD design curve method is proposed. It is confirmed that residual stress effects on brittle fracture strength in a low fracture toughness region are fairly well estimated using a proposed method by comparing estimated brittle fracture strength with experimental one.