Abstract: The electron beam welding technique was used to weld the 33 mm-thick TC4 titanium alloy. A welded joint,in good surface appearance and no internal deflects,was obtained by the proper welding parameters. Microstructures,microhardness and mechanical properties of the joint were investigated at room temperature.Results show that,in the welding seam and the heat affected zone,the martensite structure is composed of β transformation and the α'. The grains in the welding seam are relatively coarse,while in the HAZ,the grains are fine and uneven distributed. The microhardness in the welding seam is slightly lower than that in the base metal,and the microhardness is uniform in the welded joint,indicating there is no weakened area in the joint. The tensile strength is up to 956 MPa,and the average impact value of the welded metal reaches to 56 J,showing an excellent toughness in the seam.