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700°C class A-USC (Advanced Ultra Super Critical) thermal power plant is one of the solutions to reduce carbon dioxide (CO2) emission. In order to meet the design requirement, which is capable to be operated under 700°C steam condition, the development of Ni-based superalloys is essential. Fundamental research and demonstration of mock-up plants have been driven by national project in Japan, US, Europe and China, to confirm the reliability of Ni-based superalloys. One issue of superalloys reported by others is the stress accelerated grain boundary oxidation (SAGBO), leading to the deterioration of creep rupture properties during service. We newly manufactured the test apparatus which enables to creep test under high temperature steam environment, and evaluated the grain boundary oxidation behavior of Ni-based rotor material, TOS1X-2, under various stresses and atmosphere. Microstructure observation after creep tested at 750°C for 2,500h, found that thin layer of Cr-rich and Al-rich oxides form at all stress and atmosphere condition. Detail studies revealed that the Al-rich oxide were formed at fine, recrystallized grain boundary, which is driven by initially induced strain by machining. The quantitative analysis of microstructure showed that steam atmosphere slightly accelerates the grain boundary oxidation. On the contrary, effect of applied stress on oxidation depth was not observed. The grain boundary oxidation observed in this study was very small, suggesting that TOS1X-2 exhibits good oxidation and creep rupture properties in 700°C A-USC operating condition.