Microstructure　and　phase　transformation　of　Ti46Al8Nb0.5B0.2C　alloy　have　been　investigated.　X-ray　diffraction　(XRD),　optical　microscopy　(OM),　scanning　electron　microscopy　(SEM)　and　transmission　electron　microscopy　(TEM)　results　show　that　as-cast　and　hot　isostatic　pressing　(HIP)　alloy　mainly　composed　of　gamma　and　alpha(2)　phase　have　fully　lamellar　microstructure　with　point-like　or　ribbon-like　TiB2　distributing　in　lamellar　colony　or　at　grain　boundary.　The　mean　size　of　lamellar　colony　is　about　150　and　450　mu　m　for　as-cast　and　HIP　alloy,　respectively.　The　lamellar　spacing　is　about　550　and　600　nm　for　as-cast　and　HIP　alloy,　respectively.　It　has　been　found　that　cooling　rates　and　quenching　temperatures　have　significant　effect　on　phase　transformation　of　Ti46Al8Nb0.5B0.2C　alloy.　When　the　alloy　is　treated　at　1380　degrees　C　for　1　h　and　cooled　from　alpha　domain,　water　cooling　leads　to　complete　alpha　-＞　alpha(2)　transformation,　oil　cooling　leads　to　predominant　alpha　-＞　alpha(2)　and　part　alpha　-＞　gamma(m)　transformation,　air　cooling　leads　to　alpha　-＞　alpha　+　gamma(p2)　-＞　L(alpha　+　gamma)　-＞　L(alpha(2)　+　gamma)　transformation,　and　furnace　cooling　leads　to　alpha　-＞　alpha　+　gamma(p3)　-＞　L(alpha　+　gamma)　-＞　L(alpha(2)　+　gamma)　transformation.　However,　when　the　alloy　is　treated　at　1400　degrees　C　for　1　h　and　cooled　from　a　domain,　water　cooling　leads　to　predominant　alpha　-＞　alpha(2)　and　part　alpha　-＞　alpha　+　gamma(p4)　-＞　gamma(n)　transformation,　oil　cooling　leads　to　alpha　-＞　alpha　+　gamma(p5)　-＞　gamma(m)　transformation,　air　cooling　leads　to　alpha　-＞　alpha　+　gamma(p6)　-＞　L(alpha　+　gamma)　-＞　L(alpha(2)　+　gamma)　transformation,　and　furnace　cooling　leads　to　alpha　-＞　alpha　+　gamma(p7)　-＞　L(alpha　+　gamma)　-＞　L(alpha(2)　+　gamma)　transformation.　Microstructural　evolution　of　the　alloy　during　various　heat　treatments　has　been　examined　and　the　phase　transformation　mechanisms　have　been　elucidated.　Based　on　the　experimental　observation,　schematic　CCT　diagrams　for　the　alloy　have　been　given.