Oxidation resistance of TiAl3, one of the candidates of coating materials for high temperature structural materials such as Ti3Al and TiAl, has been studied. Specimens were prepared by forming TiAl3 in Al/Ti/Al reaction diffusion couples at 923 K and then TiAl3 layer was exposed to air by dissolving Al plate in a 1N NaOH solution. The obtained TiAl3/Ti/TiAl3 couples were annealed in air in the temperature range from 1173 K to 1468 K. The oxidation rate was compared with that determined by using bulk TiAl3. The present data show a bend on the Arrhenius plot of parabolic phase growth rate constant, k2, at 1323 K. Above 1323K, the constant coincides well with the extrapolated values of bulk data while the value in the lower temperature range is larger than that of bulk specimens. During the oxidation experiments, intermetallic compounds Ti3Al, TiAl and TiAl2 were formed between Ti and TiAl3. Interdiffusion coefficients in the Ti3Al, TiAl phases determined from these diffusion couples are more than one order of magnitude larger than the interdiffusion coefficients determined by previous investigators from single-phase diffusion couples but coincide with the coefficients determined from multi-phase diffusion couples. This difference between interdiffusion coefficients has been discussed and explained by the effect of boundary diffusion in the diffusion layers formed in the multi-phase diffusion couples.