Abstract: Electron beam welding of 23Co14Ni12Cr3MoE ultra-high strength steel (A-100 steel) was realized. Effect of electron beam welding on mechanical properties and microstructure of A-100 steel was studied. The result showed that after heat treatment (gas quenching), microstructure of A-100 steel forgings’ weld directly melted by electron beam was mainly lath martensite, which was staggered and distributed into basket shape. Microstructure of weld presented a certain degree of non-uniform, martensite in the middle was coarser, residual austenite was less. Residual austenite in heat affected zone increased and there was a large number of thin film reverse austenite. Strength and plasticity of electron beam weld of A-100 steel were basically equivalent to those of matrix. Notch sensitivity of weld and heat affected zone was small and notch sensitivity coefficient was about 0.6. Impact toughness of weld decreased to 2/3 of matrix, while impact toughness of heat affected zone rise to more than 80% of matrix. High cycle fatigue limit of weld could reach more than 700 MPa and weld had good fatigue properties.Highlights: A-100 ultra-high strength steel was welded by electron beam welding technology. Microstructure and mechanical properties of weld after preparation heat treatment + welding + final heat treatment and matrix after preparation heat treatment + final heat treatment were compared and analyzed.