A　TC4-DT　titanium　alloy　was　first　subjected　to　different　solution　treatment　and　then　effect　of　electron　beam　welding　on　the　microstructure　of　welded　joint　was　investigated.　The　results　show　that　a　duplex　structure　of　primary　α-phase　and　β-phase　transformed　structure　is　formed　in　the　alloy　solution-treated　at　960°C　under　furnace　cooling.　The　microstructure　is　fine　and　the　weld　contour　is　clear.　The　heat　affected　zone　(HAZ)　is　composed　of　equiaxed　primary　α-phase　and　needle　martensite　α′　phase　and　basket-like　martensite　α′　phase　is　observed　in　the　fusion　zone(FZ).　After　solution　treatment　at　1018°C,　a　coarse　grained　lamellar　Widmanstatten　microstructure　forms　and　the　HAZ　consists　of　lath-like　α-phase　and　needle　martensite　α′　phase.　The　width　of　FZ　with　column　grains　increases　and　the　weld　contour　becomes　unclear.　For　the　alloy　solution-treated　at　960°C+1018°C,　the　size　of　Widmanstatten　microstructure　is　biggest　and　the　width　of　FZ　with　column　grains　is　widest.　Under　air　cooling　condition,　the　thickness　of　α　lamellae　both　in　base　metal　and　HAZ　is　thinner　than　that　at　furnace　cooling　condition,　however,　the　martensite　α′　phase　in　FZ　becomes　thicker　and　wider.