[en] A large number of fracture toughness tests were performed in order to assess the use of the circumferentially-Cracked Round Bar (CRB) as a potential method for the measurement of fracture toughness of Reactor Pressure Vessel steels. Test conditions were selected to: (1) characterise fracture toughness in the transition region; (2) study the size effect and loss of constraint; (3) establish the limit of validity of this geometry; (4) investigate the ductile fracture at the upper shelf. In the transition region, the fracture toughness obtained from the CRB over-estimates the actual value as long as the loss of constraint and size effect were not taken into account. In addition, the B1/4 size correction is verified and gives a very good description of the size effect. The application of these corrections allows a good prediction of the normalised fracture toughness up to high levels of fracture toughness.In the upper shelf region, promising results were obtained with this geometry to characterise the ductile crack initiation and propagation