In order to comprehensively assess creep damage of 18Cr-9Ni-3Cu-Nb-N steel (ASME SA-213 S30432), which is widely used in critical high-temperature regions of heat transfer tubes of ultrasupercritical (USC) boilers, our investigation centered on the σ phase. This phase undergoes formation and coarsening during prolonged thermal exposure. We developed a technique to estimate operational heating metal temperatures by analyzing average particle size of the σ phase (MLAS-EX). By extracting a certain number of σ phase from the largest particle size, it is possible to select the σ phase that nucleated and grew in the early stage of heating. The correlation between the average particle size and the Hollomon-Jaffe Parameter (HJP), a parameter of heating temperature and time, allows precise estimation of the heating metal temperature. Our validation demonstrates that the replica method, which is a nondestructive method and effective for evaluating actual plants, is also applicable. Using our newly developed technique for estimating heating metal temperature, it is possible to predict the remaining creep life of heat transfer tubes based on data related to creep rupture characteristics, working stress and operating time. The developed method has already been successfully applied to evaluate the creep life of several actual boilers.