Since the British Standards Institution (BSI) standardized BS 5762 in 1979, a popular way of calculating the Crack Tip Opening Displacement (CTOD) has been the use of the plastic hinge model with an assumed rotational centre. The American Society for Testing and Materials (ASTM) previously accepted the plastic hinge model and standardized E1290 in 1989. However, ASTM revised E1290 in 2002, and has proposed a fracture parameter conversion from the J-integral to CTOD. These two different CTOD calculations probably lead to confusion for Fitness-for-Service (FFS). The Fracture Toughness Study Committee of the Japan Welding Engineering Society (JWES) organized a working group, and the effects of CTOD testing methodology on CTOD values were investigated. In this paper, the results of CTOD round-robin tests in the working group are summarized, and the difference between the critical CTOD values obtained by ASTM E1290–02 and those by BS 7448, which involves the plastic hinge model, is described.