[en] A study has been conducted to survey the most suitable steel as the temperature monitor being used for the radiation damage experiments in nuclear reactors and to develope the new technique to estimate the temperature more accurately. Eighteen kinds of steels have been aged at temperatures from 375 to 700₀C for various periods of time (10 -- 3,000 hrs), and a hardness has been measured by a Vickers hardness tester. It has been concluded that JIS SUJ2 (ASTM52100) is the most suitable one as the temperature monitor. It could give a wider available temperature range (375 -- 700₀C) and a more accurate estimated temperature (confidence interval <= 26₀C) than the conventional Templugs (BS EN52, BS1407). It has been shown that a micro-Vickers hardness tester (1 kgf) weight) can be used as the substitute for a Vickers hardness tester. (author)