High temperature cyclic oxidation behavior of γ'-base Ni-25Al-10Pt (in at.%) alloy was investigated at 1000°C in air with and without 30vol.%H2O. The oxidation mass gain during the initial stage of oxidation was similar in both atmospheres, but the oxidation rate in air+H2O was lower in the longer steady-state oxidation stage. Metastable Al2O3, which formed during the initial stage of oxidation, transformed completely after about 100hr of oxidation in dry air. The transformation to α-Al2O3 also occurred in air+H2O, but complete transformation to α-Al2O3 was not observed during the oxidation time in the present study. θ-Al2O3 grains remained for longer on the α-Al2O3 layer in air+H2O and became significantly coarser with oxidation time. The present results indicate that water vapor delays the metastable to α-Al2O3 phase transformation, and decreases the growth rate of α-Al2O3.