Al-rich aluminide coating on Ti-49.1at%Al alloys has been performed by two-step process of thermal aluminum spray and diffusion treatment. Effect of the diffusion time on the oxidation resistance, and the change of microstructures in the coating during the oxidation test in air at 900°C for 100 h were investigated. In the aluminized coating before the oxidation tests, formations of TiAl 3 on the outer layer and an intermediate layer consisting of Ti 2Al 5, TiAl 2, and Al-rich TiAl were observed. The intermediate layer developed by the diffusion treatment for 3600s. In the oxidation test over 50 h, the specimen diffusion treated for 3600 s exhibited the lower oxidation rate than that diffusion treated for 900 s. Multi-oxide scales of TiO 2 and Al 2O 3 were observed on the both surfaces of aluminized specimens. Large TiO 2 particles were observed on the specimen of t D = 900 s. It was found that the TiAl 2 layer developed during the oxidized specimen, while the thicknesses of Al- rich TiAl and TiAl 3 layer decreased by the growth of TiAl 2. © (2012) Trans Tech Publications, Switzerland.