The brittleness of TiAl3 is improved by the addition of third elements such as manganese, silver, chromium and so on. The effects of the third elements on oxidation resistance of TiAl3 have been investigated in the point of view of the microstructure and products of oxidation layer. The oxidation test was carried out at 1000°C for 25 h or 30 d in air. For the copper-substituted alloy, Ti(Al, Cu)3, the oxidation layer consisted of TiO2, whose microstructure was porous. Thus Ti(Al, Cu)3 showed very low oxidation resistance. While, for the manganese, silver, iron or chromium-substituted alloys, these oxidation layers consisted of only Al2O3. These alloys showed the oxidation resistance. It is not expected that the iron- or chromium-substituted alloys show the oxidation resistance of long duration, because oxygen atom was detected by EDS analysis in Al-depleted layer after the oxidation test for 30 d. In the oxidation test at 1000°C, the oxidation layer of the iron- or chromium-substituted alloys was constituted of α-Al2O3 and θ-Al2O3. On the other hand, that of the manganese- or silver-substituted alloys was constituted of only α-Al2O3 and their oxidation layer is dense. From these results, it was understood that the substitution elements would influence on the temperature of θ→α transformation. That is, Fe and Cr would solute to θ-Al2O3, and then θ→α transformation temperature would increase. While, Mn and Ag would scarcely solute to θ-Al2O3, and then θ→α transformation temperature would be almost the same as that of pure θ-Al2O3. In the oxidation test at 1000°C, the residence of θ-Al2O3 decides oxidation resistance of Ti(Al, X)3 materials. The choice of the substitution element promoting the transformation from θ-Al2O3 to α-Al2O3 during heating process is important for improving in the oxidation resistance.