Abstract: Electron beam welding was used to weld 18 Ni Cofree maraging steels.The microstructure of welded joints was investigated by optical microscopy, and the microhardness of weld metal, heat-affected zone and base material was tested.The results show that the microstructure of the base materials is lath martensite, and the microstructure of weld metal exhibits a typical cystiform-dendritic morphology.The region adjoining the weld metal was heated to high temperatures in the austenite phase before rapid cooling.EB welding heat input was high enough to lead to grain recrystallization.The softest region is weld seam, while the hardest is fine-grained zone. the microhardness in the heat-affected zone increases with the distance being away from fusion line.