The　100　mm　thickness　SUS304　stainless　steels　are　welded　by　the　ultra-narrow　gap　laser　welding,　and　the　microstructures　and　mechanical　properties　of　the　joints　are　analyzed.　The　results　show　that　the　hundred-millimeter-grade　thickness　plates　can　be　effectively　welded　by　the　ultra-narrow　gap　laser　welding　and　the　welds　present　good　surface　appearances.　The　microstructures　of　the　joints　are　composed　of　austenite　and　small　amount　of　ferrite,　and　the　grains　are　with　the　cellular　equiaxed　shapes.　The　tensile　strength　of　the　joints　is　658　MPa　and　the　tensile　fracture　is　located　at　the　cellular　grain　zone　near　the　fusion　line.　The　fracture　morphology　shows　fine　equiaxed　dimples　and　tearing　ridges,　which　indicates　that　the　fracture　is　ductile.　The　microhardness　at　the　equiaxed　grain　zone　is　obviously　higher　than　that　at　the　cellular　grain　zone.　©　2018,　Chinese　Lasers　Press.　All　right　reserved.