Steam oxidation behavior of a new Ni-based single crystalline superalloy (TMS-75) developed as the high strength material for the advanced gas turbine which would be cooled by steam was studied in comparison with Inconel 625 alloy which is a representative Ni based superalloy and 12Cr ferritic steel which is conventionally used in boiler tubes and main steam pipes and steam turbines. TMS-75 alloy showed worse oxidation resistance in steam than Inconel 625 alloy, but it had much better oxidation resistance in steam than 12Cr steel. Also, the oxide scale of Inconel 625 alloy is very thin and is mainly composed of Cr2O3. However, the outer scale of TMS- 75 alloy was mainly composed of NiO with Co and a little Al and other elements. The Ni/Co ratio in the oxide scale was a little higher in the outer layer than in the inner layer.