The relationship between the average elastic stress concentration factor, Kt and the stress intensity magnification factor, MK, was theoretically indicated, and the applicability of MK to the WES 2805 CTOD design curve was investigated by using the results of finite element analysis for four welded joints. In addition, the average local strain by area around an assumed crack was calculated by the analysis, and the strain was used as the local strain, ε, in the WES 2805 CTOD design curve. It was demonstrated that MK was helpful in calculating accurate Kt, which contributes to a good CTOD estimation using the WES 2805 CTOD design curve. The average local strain also contributes to a good CTOD estimation, especially for a toe crack in the high strain gradient region. These two options, the use of MK and the average local strain, were approved by the Fracture Toughness Study Committee of the Japan Welding Engineering Society, and were adapted for WES 2805 revised in 2007.