The isothermal oxidation behavior of intermetallic alloys over a wide range of compositions in the Ti-Al-Nb system was investigated in a temperature range from 900 to 1,200°C. The results showed that the Al content and the ratio of Ti-to-Nb of the alloys affected the oxidation behavior in this ternary system. Al improved the oxidation resistance of alloys. If the ratio of Ti-to-Nb was either too high or too low, the oxidation resistance was reduced dramatically. The oxide constituents were determined. The formation of an integral protective Al2O3 scale layer was the basic condition for good oxidation resistance. A composition range of the alloys, having excellent oxidation resistance, was suggested. A map summarizing the results of the oxidation resistance and the oxide scale type was established. The oxidation mechanism was also discussed.