The oxidation behavior of NiTi and NiTiNb alloys containing different amounts of Nb (7%, 9%, mole fraction) were studied at 800 ℃ in air. It is found that the oxidation resistance of NiTi alloy can be effectively increased by the Nb addition. Under the same oxidation condition, the mass gain of NiTi is about 7 mg/cm2, while the mass gains are only 3 mg/cm2 for Ni47Ti44Nb9 alloy and 2.4 mg/cm2 for Ni52Ti41Nb7. Moreover the oxidation resistance of single phase NiTiNb alloy is better than that of the dual-phase alloy with large amount of Nb precipitates. On the basis of thermodynamics and kinetics of oxidation, the effect of Nb alloying element on the oxidation behavior of NiTi-based alloys was discussed.