Obtaining　a　fundamental　understand　of　the　relationship　between　weld　microstructure　and　mechanical　properties　is　critical　for　ensuring　a　high　weld　quality.　This　work　explored　the　micro-mechanism　responsible　for　the　mechanical　properties　of　a　10-mm-thickness　metastable　β　titanium　alloy　(Ti–3Al–5Mo–4Cr–2Zr–1Fe,　wt.%)　joint　welded　by　electron　beam　welding　(EBW).　Weld　microstructure　and　its　evolution　behavior　were　revealed　by　means　of　multi-scale　microstructure　characterization.　Obtained　results　indicate　that　the　absence　of　α　phase　and　formation　of　nano-scale　ω　particles　occurred　in　the　coarse　columnar　β　grains　of　the　fusion　zone　(FZ).　Intense　dissolution　and　significant　coarsening　of　secondary　α　phase　occurred　in　the　heat　affected　zone.　Between　the　FZ　and　HAZ,　a　partially　melted　zone　was　observed　and　contains　few　equiaxed　β　grains.　Compared　to　the　base　material,　the　as-welded　EBW　joint　shows　significant　softening　in　weld　seam　and　a　slight　decrease　of　17%　in　ultimate　tensile　strength,　while　a　severe　reduction　of　73%　in　elongation.　It　was　observed　that　majority　of　plastic　deformation　is　confined　to　the　narrow　FZ,　indicating　a　significant　localization　of　tensile　strain.　This　leads　to　premature　fracture　of　the　joint　within　the　FZ　and　consequently　to　significant　deterioration　of　the　overall　ductility.　This　work　contributes　to　advancing　the　understanding　of　the　role　of　microstructural　evolution　on　the　mechanical　properties　of　high-strength　titanium　alloy　joints　via　EBW.　©　2024　The　Authors