Applying　the　longitudinal　magnetic　field　to　the　process　of　streaming　transfer　MAG　welding　with　98%Ar+2%O2　and　80%Ar+20%CO2　shielded　gases,　the　influence　of　the　longitudinal　magnetic　field　on　arc　shape　and　motion　character　is　researched　by　means　of　a　high　speed　camera.　It　is　revealed　that　the　essential　effect　of　the　outer　longitudinal　magnetic　field　on　MAG　welding　arc　is　arc　constriction.　Through　analyzing　the　force　of　the　liquid　metal　on　the　end　of　liquid　metal　jet,　the　drop　transfer　mechanics　under　the　effect　of　the　longitudinal　magnetic　field　is　proposed.　The　result　shows　that　the　application　of　the　longitudinal　magnetic　field　can　change　arc　shape　from　pyramidal　and　static　to　spiral　and　rotational　with　high　speed.　With　the　increment　of　exciting　current,　the　angular　velocity　of　arc　is　accelerated　and　arc　length　is　shortened　and　electric-field　intensity　is　raised.　Also　the　application　of　the　longitudinal　magnetic　field　can　decrease　the　welding　current　and　increase　the　transfer　frequency　of　molten　drop　and　the　melting　coefficient　of　wire.　The　stability　of　MAG　welding　process　is　affected　by　the　outer　longitudinal　magnetic　field　and　the　physics　nature　of　shield　gas.　©　2010　Journal　of　Mechanical　Engineering.