Electron　beam　welding　(EBW)　was　applied　to　50　mm　thick　damage-tolerant　Ti-6Al-4V　(TC4-DT)　alloy,　and　microstructure,　microhardness　and　tensile　properties　of　the　defect-free　welded　joints　were　examined.　The　results　indicated　that　the　microstructure　of　the　base　metal　is　composed　of　primary　alpha　phases　and　the　lamellar　(alpha　+　beta)　bimodal　structure.　For　the　EBW　joint,　martensite　basketweave　microstructure　is　formed　in　fusion　zone　(FZ).　Moreover,　the　heat　affected　zone　(HAZ)　near　FZ　consists　of　acicular　martensite　and　a　small　portion　of　primary　alpha　phase.　The　HAZ　near　base　metal　consists　of　primary　alpha　phase　and　transformed　beta　containing　aciculate　alpha.　It　is　found　that　the　boundary　of　the　two　portions　of　the　HAZ　was　dependent　on　the　p　phase　transus　temperature　during　weld　cooling.　Microhardness　values　for　FZ　and　HAZ　are　higher　than　that　of　base　metal,　and　there　are　the　peak　values　for　the　HAZ　near　the　weld　metal.　The　fracture　locations　of　all　the　EBW　tensile　specimens　are　in　base　metal,　and　the　ultimate　tensile　strength　of　the　joints　may　reach　about　95%　of　the　base　metal.　In　addition,　with　the　depth　increasing　along　the　weld　thick　direction,　the　grain　size　of　the　FZ　decreases　and　microhardness　increases.　(C)　2011　Elsevier　Ltd.　All　rights　reserved.