Abstract: The pressure to temperature (P-T) curve is significant to ensure the vessel integrity and deal with the reactor pressure vessel(RPV) ageing. The traditional method using the material near static facture limit value (K_Ic) by t-R_TNDT curve to plot the P-T curve cannot obtain the appropriate R_TNDT for an irradiated material and is overly conservative. With the surveillance data of a reactor pressure vessel, this paper calculates the Master Curve index temperature, R_TT0, at the ending of the RPV 50 years life. The ASME Code Case N629 effort to utilize the Master Curve Method is used to plot the P-T curve , compared with results of traditional method, the P-T operating window is extended and the RPV economy is improved.