Abstract: Pore defects are the major problem in vacuum electron beam welding. The inducement mechanism and thermodynamic equilibrium mechanism were analyzed from the perspective of force balance condition, and the minimum radius survival conditions for pores in molten pool were proposed. The results showed that the gasification nucleus is an important factor induced bubbles in molten pool from the perspective of bubble dynamics. The bubbles, which don't meet the minimum radius survival condition, do not satisfy the thermodynamic equilibrium condition and therefore can’t develop into the pores. The size of the bubbles generated on the wall of middle part of weld depends largely on the gas density in bubble, surface tension of bubble, and overheating of gasphase components in bubble. The overheating is not only a driving force for bubble stability, but also a main factor determining the bubble size.