Microstructure and high temperature oxidation behaviour of Ti45Al8Nb0.5B0.2C alloy was investigated in this work. XRD, SEM, TEM and EDS analysis demonstrate that as-cast Ti46Al8Nb0.5B0.2C alloy after HIP has fully lamellar microstructure consisting of γ and α2 with point-like or ribbon-like TiB2 distributing in the colony or at grain boundary. When the alloy was treated at 1400 ℃ for 1 h and oil cooled from the single α filed leads to α→γm transformation and γm was aged in the α+γ two-phase field, alpha plates precipitated on all {111} planes of the γm to form fine microstructures. In addition to determination of the oxidation knetics by mass gain measurements, surface and cross-sectional morphologies of oxidizing layer formed was observed by SEM and the present phases in the scales were analyzed by XRD. Results show that oxidation reaction constant and power index for Ti46Al8Nb0.5B0.2C alloy oxidized at 900 ℃for 100 h were 0.00192 and 1.1381, respectively. However, oxidation reaction constant and power index for Ti46Al8Nb0.5B0.2C alloy oxidized at1000 ℃ for 100 h were 0.31637 and 2.0076, respectively. Oxidizing layer of Ti46Al8Nb0.5B0.2C alloy after 900℃, 100 h oxidation was about 4 μm and composed of four layer. The first layer is discontinous Al2O3, the second layer is the mixture of Al2O3 and TiO2, and the last is rich in Nb. Oxidizing layer of Ti46Al8Nb0.5B0.2C alloy after 1000 ℃, 100 h oxidation was about 5 μm and composed of 4 layer. The first layer is TiO2, the second layer is the mixture of Al2O3 and TiO2, and the third layer is nitride layer in sequence.