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The initial oxidation behavior of TiAl-Nb alloys was systematically investigated against the composition, temperature, and partial pressure of O₂ with the CALculation of PHAse Diagrams (CALPHAD) technique. The mole fraction of each oxidation product at the initial oxidation stage of the alloys at the corresponding temperatures was predicted. The initial oxidation products of the alloys are oxides of Al, Ti, and Nb, with the oxidation order of Al, Ti, and Nb. As P(O₂) increases, the Ti and Nb oxidation products move towards a high oxygen content, and the mole fractions of the Al and Ti oxides gradually decrease and increase, respectively. It was found that the temperature and partial pressure of O₂ determined the types of the oxides and the oxidation order, while the concentration of Nb and Al determined the mole fraction of the oxides. The CALPHAD results are in good agreement with the experiments.