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　gamma　and　alpha(2)　with　point-like　or　ribbon-like　TiB2　distributing　in　the　colony　or　at　grain　boundary.　When　the　alloy　was　treated　at　1400　degrees　C　for　1　h　and　oil　cooled　from　the　single　alpha　filed　leads　to　alpha　-＞gamma(m)　transformation　and　gamma(m)　was　aged　in　the　alpha+gamma　two-phase　field,　alpha　plates　precipitated　on　all　{111}　planes　of　the　gamma(m),　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　degrees　C　for　100　h　were　0.00192　and　1.1381,　respectively.　However,　oxidation　reaction　constant　and　power　index　for　Ti46Al8Nb0.5B0.2C　alloy　oxidized　at1000　degrees　C　for　100　h　were　0.31637　and　2.0076,　respectively.　Oxidizing　layer　of　Ti46A18Nb0.5B0.2C　alloy　after　900　degrees　C,　100　h　oxidation　was　about　4　mu　m　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　degrees　C,　100　h　oxidation　was　about　5　mu　m　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.