In the concept of the damage tolerance design, the defects are thought to be acceptable when the conditions to cause failure are not reached. By this principle, a practical acceptance standard has been established in Japan which provides the assessment method for weld defects, based on the COD concept. But this standard has several aspects to be improved particularly for lack of careful consideration about probabilistic nature of failure phenomena. Many factors such as applied load, toughness of the material, fatigue crack growth and non-destractive methods contribute to the probabilistic aspects of failure. In this paper, one of those parameters, namely, toughness of the material is considered. The purpose of this study is to clarify the uncertainties included in the present procedure for the assessment of material toughness 5, and to propose a modified procedure based on reliability analysis.