Experiments　of　electron　beam　welding　was　carried　out　for　a　50　mm　thick　TC4-DT　titanium　alloy.　The　effect　of　electron　beam　welding　on　the　microstructure　characteristics　was　investigated　using　optical　microscope　observation　and　microhardness　measurement　for　the　cross-section　of　the　welded　joints.　The　results　show　that　the　microstructure　of　base　metal　was　typical　bimodal　structure　composed　of　primary　alpha　phase　and　lamellar　(alpha+beta).　The　microstructure　of　weld　metal　was　basket-like　martensite　alpha＇.　Coursed　primary　beta　grain　boundaries　were　clear　in　upper　and　middle　parts　of　weld　metal,　while　the　size　of　the　beta　grains　was　small　and　the　grain　boundaries　were　not　obvious　in　the　bottom.　The　microstructure　of　heat　affected　zone　(HAZ)　may　be　divided　into　two　regions.　The　microstructure　of　HAZ　near　fusion　zone　was　consisted　of　little　primary　alpha　phase　and　needle　martensite　alpha＇,　while　the　microstructure　near　base　metal　was　constituted　of　primary　alpha　phase　and　transferred　beta　containing　aciculate　alpha.　The　boundary　of　the　two　regions　was　dependent　on　the　beta　phase　transfer　temperature　during　weld　cooling.　The　microhardness　of　the　weld　metal　and　HAZ　was　higher　than　that　of　base　metal,　and　there　was　a　peak　value　of　microhardness　in　the　HAZ　near　fusion　zone.　In　addition,　the　microhardness　of　the　fusion　zone　had　an　increased　tendency　with　the　measured　weld　depth　increasing.