Abstract: Electron beam welding of TC21 56 mm titanium alloy was carried out.The microstructure and the mechanical properties of welded joints were analyzed and tested.The results showed that the weld zone consisted of the columnar β grains, and in which the transgranular acicular α' martensite were dispersedly distributed.HAZ can be divided into three parts from base metal to weld zone, which are the equiaxed recrystallized β grain zone, Widmanstaten structure zone formed by lamellar and acicular α phases and lamellar α phase coarsening zone.Fusion zone consists of the adnate columnar and equiaxed grains.Tensile strength of joints reaches to that of base metal and the failure appears in the base metal.The mechanical properties are uniform along the vertical direction.Plasticity in the welded joint is greatly decreased and only up to 50% of that of the base metal.The microhardness in weld zone is the highest, and that of the equiaxed grain zone and Widmanstaten structure zone in HAZ is higher, and the microhardness in columnar α phase coarsening zone is the lowest.