The threshold characterization for penetration welding of A3 steel was investigated using a Nd:YAG and a CO2 laser with high speed camera and optical spectrometer. It is shown that the threshold ratio of the laser power to the beam spot diameter P/d for penetration welding is consistent with different beam spots. But the threshold power density changes with the beam spots. The transition threshold of welding mode of YAG laser is larger than the vaporization threshold of materials. The metal vaporizes obviously even in the period of conduction welding and the spectra of the vapor plume remain the same between the conduction and penetration welding modes in YAG laser welding. As for CO2 laser welding, the metal vaporizes weakly before the mode transition occurs from conduction welding to penetration welding, and the metal vaporizes violently and the vapor ionizes partly in penetration welding. Analysis shows that the formation of a vapor plasma enhances the absorption of metal to laser beam and thus promotes the establishment of the penetration welding process in CO2 laser welding.