The　influence　of　B　and　Y　on　the　microstructure,　microsegregation,　and　tensile　behaviour　of　the　Ti45A18Nb0.2W0.25Cr　(at%)　alloy　was　investigated.　The　beta-stabilizer　elements　in　the　high-Nb　TiAl　alloy　promote　the　formation　of　the　gamma　phase　in　the　microsegregation　region　and　lead　to　the　formation　of　large　blocky　microsegregation　areas.　The　large　blocky　microsegregation　regions　with　low　specific　surface　areas　reduce　the　nucleation　rate　of　cavities　and　cracks　at　the　interfaces　of　the　microsegregation,　which　are　harmful　to　colony　boundary　strengthening　and　decrease　tensile　resistance.　The　addition　of　B　and　Y　affords　an　obvious　refinement　in　the　lamellar　colony,　renders　an　increasing　opportunity　for　cavity　nucleation　at　the　colony　boundary,　and　thus　improves　the　tensile　resistance.　The　tensile　mechanisms　of　the　alloys　before　and　after　(B,　Y)　addition　were　also　compared　and　analysed.