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.