Abstract: Through observing microstructure of electron beam welding of molybdenum and niobium monocrystal alloy, it is seen that the stress inflicted by machine or welding can induce subcrystal, twin crystal and flaw in monocrystal welding process. Based on the theoretic analysis for engendered these welding defects, the way how to avoid the defects inflicted by stress coming from outside during welding was proposed.