Silicon diffusion coating has been prepared on TC4 alloy by pack cementation technique to improve its high temperature oxidation resistance. The microstructure, high temperature oxidation behavior and failure mechanisms of the coating were studied. The results show that the coating has a dense multilayer structure, mainly consisted of a TiSi₂ outer layer, a TiSi middle layer, and a Ti₅Si₄+Ti₅Si₃ inner layer. The high temperature oxidation tests show that a protective scale composed of SiO₂ and TiO₂ formed on the surface of the coating during oxidizing at 850 ℃ in air, imposed to relatively good anti-high temperature oxidation property of the coating. The inter-diffusion of Ti and Si between the coating and the substrate caused the increase of TiO₂ in the scale and lacking of Si source, resulting in the degeneration of the protective scale. The high P-B ratio between the oxides and the coating, together with the mismatch of the thermal expansion coefficients among the oxides and the coating should be the main factors responsible for cracking and spallasion of the scale.