Background: The leak test temperature and startup and shutdown pressure-temperatue (P-T) limits for reactor pressure vessel are specified in the ASME code and in the U.S. Part 50 to Title 10 of the Code of Federal Regulations. The ASME Code Case N-629 has been integrated into the 2013 ASME code. The KIc curves indexed by RTNDT and RTT0 are both accepted by the ASME code. Purpose: The application of KIc-RTT0 curve to the determination of the P-T curve needs to be compared against the results using KIc-RTNDT and KIR-RTNDT curves. Methods: The P-T limit curves at the design life of 40 years and in the extended period are made for a home-made Reactor pressure vessel (RPV) which is taken as an example. Results: It is indicated that there exists great difference among results using the three methods. The P-T operating window based on KIc-RTT0 is widened. The brittle facture can not occur during the normal start-up and cool-down transients. The P-T limit curves from KIc-RTNDT curve are relatively conservative while the results based on KIR-RTNDT curve are too conservative. Conclusion: The analysis in this paper provides the technical support to the operation and the potentional of life extension for this RPV. Abstract