Laser-micro-welding　based　on　galvanometer　scanning　has　attracted　great　interest.　Herein,　bead-on-plate　scanning　micro-welding　of　100-μm-thick　AISI304　stainless　steel　foil　is　conducted　by　using　a　single-mode　fiber　laser　equipped　with　a　galvanometer　scanning　system.　A　comparative　study　is　conducted　on　the　laser-micro-welding　processes　with/without　Ar-gas　protection,　and　process　windows　of　laser-micro-welding　for　the　both　conditions　are　established.　Results　show　that　an　unstable　transition　zone　exists　during　laser-micro-welding　without　gas　shielding.　The　unstable　transition　zone　is　generated　during　the　deep　penetration　welding　mode　alone,　rather　than　during　the　alternation　of　thermal　conductivity　welding　mode　and　deep　penetration　welding　mode,　as　reported　previously.　The　oxidation　during　laser-micro-welding　results　in　an　unstable　deep　penetration　welding　mode,　during　which　a　transition　zone　is　formed.　Furthermore,　by　applying　gas　shielding,　we　find　that　the　unstable　transition　zone　is　eliminated,　and　the　welding-process　window　is　expanded.　©　2019,　Chinese　Lasers　Press.　All　right　reserved.