The three-point bending COD test with clip gauge technique is a promising method of determining the toughness of structural steels in terms of the modern concept of fracture mechanics. The consistency of the critical COD value has been demonstrated by many investigators. The present authors supplement here some information in this aspect of problem, viz, the effect of notch acuity, the thickness insensitivity, comparison between small sized bending test and deeply notched wide plate tension test, and so on. A simple method of converting the critical COD to the value of ρ⁺ _c is experimentally established. It is supported by the fact that the critical COD divided by the yield strength is proportional to the fifth power of the absolute temperature. As an example of practical application, the welded-and-notched wide plate tension test is successfully analysed by the ρ⁺ _c concept. The ρ⁺ _c value used in the analysis is one converted from the critical COD value measured by simple testing.