Ti45Al-6Nb-xCo (x=0, 0.5, 1, 2 and4 at.%) alloys were prepared using non- consumable arc melting method by three times of remelts. The microstructure and high temperature oxidation behavior of the alloys with different Co concentrations were investigated. The results show that increase of Co concentration in the alloy can significantly refine the α₂+γ lamellaer, promote the formation of γ and B₂ phases while inhibit the formation of α₂ phase. Co-rich precipitates and microstructure coarsening were observed when concentration of Co is higher than 2 at.% in the alloy. The static oxidation results at 1000 ℃ in air show that the oxidation resistance of TiAl-Nb alloy has been remarkably improved by appropriate Co alloying. The oxidation parabolic rate constant of the Ti45Al-6Nb-1Co alloy is about 1.36×10^_-2mg^₂/cm^₄h, lower than that of the Ti45Al-Nb alloy by about one order of magnitude. However, once the concentration of Co exceeds 2 at.%, increase of Co concentration in the alloy lead to an increase of the oxidation rate of the alloy.