Other than the brittle failure, the ductile behavior of the aged nuclear reactor pressure vessel (RPV) steels is also of interest for the integrity assessment and evaluation of the irradiation response. The fracture toughness of high toughness materials like RPV steels can be characterized by a J-R curve. Since the RPV steel material available for testing purposes, like surveillance specimens, is limited, the single specimen method is used for the J-R curve determination. In this study, J-R curves were measured on Charpy size SE(B) and 1T-C(T) specimens of different RPV steels in the unirradiated and irradiated conditions. It was observed that despite the available sophisticated instrumentation and strict implementation of the recommended test procedures, the J-initiation value for all the different material specimens tested could not be ascertained according to the test standards ASTM E1820 and ISO 12 135. For Charpy size SE(B) specimens, it was found that though valid JIC/J0.2BL values could be obtained in irradiated conditions, in un-irradiated conditions, especially for high toughness RPV steels, it was not possible. The evaluation showed that the aoq fit of the ASTM standard compensates uncertainties in the initial J-Δa value resulting in reliable and more number of qualified test results. But these uncertainties strongly influence the A parameters of the ISO fit and the J0.2BL(B) value. Additionally, in the ISO evaluation the lower offset of the first exclusion line and a higher slope results in lower J0.2BL values compared to the ASTM analysis. Furthermore, for the two specimen geometries the course of J-R curves up to the JQ value was similar even for high toughness materials, but the lower specimen size was disqualified due to the lower prescribed Jlimit. Similarly, the J-R curves for un-irradiated and irradiated condition had a similar course up to the J0.2BL value, even for extremely high irradiation induced embrittlement.