With reviewing weld defects in fabricated weldments, reasonable acceptance criteria should be made so that an optimum balance of safety and economy will be assured for the weldments. In quest of the reasonable acceptance criteria, efforts are being made, as a worldwide trend, to work out some simple and practical acceptance formulae based on fracture mechanics. But the criteria are not perfect and contain a great deal to be wanted, due to the fact that the criteria were meant to be as simple as possible. The purpose of this study is to clarify the uncertainties included in the present acceptance criteria and to describe the application of reliability theory to the assessment of significance of defects with respect to brittle fracture.