Abstract: High quality welding of gear shaft is critical to improving power-to-weight ratio and service performance of helicopter transmission systems. Effect of electron beam welding process on microstructure and mechanical performance of three gear steels (9310, 16Cr3NiWMoVNbE and CSS-42L) was deeply discussed. The results showed that microstructure of three gear steel was all tiny needle martensite, grain size of weld was obviously coarse after electron beam welding, especially grain size of 9310 weld was obviously coarse. Significant change appeared in heat affected zone of CSS-42L under the action of electron beam heat, four different kinds of microstructure formed from weld boundary to base metal, appearing extremely inconsistency phenomenon. Hardness of 9310 and 16Cr3NiWMoVNbE weld was higher than that of base metals, while hardness of CSS-42L weld was lower than that of base metal. The lowest hardness zone of three kinds of welded joints was all located in HAZ, indicating that HAZ was the weakest zone of welded joints, and it was supported by mechanical test results. Tensile fracture appeared in HAZ of three EBW joints. Tensile strength and torsional strength of welded joints were slightly lower than those of base metals. Obvious necking phenomenon occurred before fracture, so plasticity reduction was little. Mechanical properties of CSS-42L were all higher than those of 9310 and 16Cr3NiWMoVNbE. Strength of CSS-42L EBW joints was higher over 50% than that of 9310 EBW joints, and plasticity exceeded over 20% than those of two gear steel EBW joints. The results provided scientific support for manufacturing of helicopter transmission systems.