The influence of thermal aging on cast stainless steels used in JAEA's advanced thermal prototype reactor “Fugen” that were exposed to 275°C for about 15 years was investigated. The degree and mechanism of thermal embrittlement were evaluated on the basis of Charpy impact test results and the three-dimensional atom probe (3DAP) analysis of the microstructural changes in the spinodal decomposition using materials obtained from Fugen. The results revealed early signs of a thermal aging effect over a long period at the low temperature.