The isothermal oxidation behavior of NiTi alloy in H₂-H₂O at the temperature range of 400 °C to 700 °C was studied. The oxidation rate followed cubic laws, and the activation energy for the oxidation of the alloy was determined to be 127.52 kJ/mol. The surface Ni content was significantly reduced by selective oxidation. For the sample oxidized at 400 °C, the morphology was different from that formed at high temperatures. For the samples oxidized at 500 °C, 600 °C,and 700 °C, the oxide scales showed two morphologies, which consisted of cubic grains and oxide whiskers. Cross-section analysis showed that the oxide scales consisted of an outer TiO₂ layer and an inner Ni₃Ti layer. Voids formed near the interface of the two layers.