The steam oxidation behavior and the oxide scale structure of Ni-based alloys containing 20 to 25 mass% Cr content were studied. The oxidation test was carried out from 550 to 700°C at 50°C intervals for 100 hours in steam. All Ni-based alloys used showed good steam oxidation resistance; higher Cr containing alloys have a little better oxidation resistance than lower Cr containing alloys. The kinetics of the oxidation rate of all Ni-based alloys used was estimated as the temperature parameter in a 100 hour test and as the temperature and time parameters, respectively. These equations could be useful for industrial applications. The oxide scale of Alloy 625 was composed of two layers. The outer layer was composed of needle-like oxides and the inner layer was composed of isometric oxides. The oxide scale was composed of Cr2O3 type. The Cr/Ni ratio in the oxide scale at 700°C was by one order larger than that at 650°C. The oxidation resistance of Ni alloys is maintained by a uniform Cr2O3 layer which is composed of high Cr content.