This paper investigates the oxidation behavior and microstructure evolution of Ti-6Al-4V alloy at 900_oC, 930_oC and 960_oC. The thickness of surface oxide layer increases during 0.5-24h and the closer to the surface, the more porous of the oxide. There is a sharp decrease of O level at the boundary of oxide layer and a-case. The O level decreases gradually inside the a-case to a stable level in the Ti-6Al-4V base material. The oxide layer is maily composed of TiO₂, and there is also small amount of Al₂O₃. The volume fraction of a in much higher in the a-case and the grain size is also larger. The relationship between a-case depth and thermal exposure time can be fitted using log-log function. Linear regression analysis shows that the activation energy of O diffision in a case is 206 kJ/mol.