A　defect-free　welded　joint　of　100　mm-thick　SUS　304　steel　plates　is　produced　by　ultra-narrow　gap　laser　welding　with　filler　wire　in　the　laser　conduction　mode　in　42　layers.　The　laser　beam　concurrent　heating　of　the　groove　side　walls　and　bottom　ensures　the　adequate　side　wall　fusion,　with　the　angle　distortion　belowed　1　degrees,　and　the　average　fusion　ratio　of　the　whole　weld　of　about　11%.　The　microstructure　is　composed　of　cellular　grains　adjacent　to　the　fusion　line　and　fine　equiaxed　grains.　The　average　tensile　strength　of　the　weld　joint　is　658　MPa,　which　corresponds　to　similar　to　95%　of　that　of　the　base　metal.　The　fracture　occurs　near　the　fusion　line　in　the　cellular　grain　zone　of　the　weld　joint.　The　microstructure　morphology　difference　in　the　weld　joint　implies　the　uneven　distribution　of　microhardness　which　values　in　the　equiaxed　grain　zone　significantly　exceed　those　the　cellular　zone.