Abstract: A study has been performed to examine the behavior of flux in electron beam welding , the result indicates that the penetration of electron beam welding can be increased by using the activating fluxes. A kind of activating flux composed of SiO2, TiO2 and Cr2O3 for welding of stainless steel is developed on base of investigation of the influencing pattern of mono flux on penetration, by which the penetration of EB welding can be increased more then two times. The formation of the weld surface is elegant. When using the flux, more important effect to the penetration increased is focus current and bind current. The reason that the flux increases the penetration lies in the following two aspects：firstly, the flux can change the surface tension gradients of weld pool then the direction of the fluid flows in weld pool been changed, thereby producing a relatively deeper and narrower weld. Secondly, the melt point of the flux is high than the metals, it reduce the melt square of the base material. On the base of above investigation bring forward the idea of activating EB welding .

Key words: Electron beam, Activating fluxes, Welding