304　stainless　steel　plates　of　60　mm　thick　are　welded　using　IPG　YLS-6000　fiber　laser　and　5　mm　wide　groove.　The　microstructure　and　mechanical　property　of　joint　are　tested　by　optical　microscope,　scanning　electron　microscope,　energy　disperse　spectroscopy　and　tensile　testing　machine　at　low　temperature.　The　result　shows　that　complete　defect　free　butt　joints　are　obtained　with　ultra-narrow　gap　laser　welding.　The　width　of　weld　is　uniform　and　is　about　3.8　mm.　Microstructure　of　joint　are　consist　of　columnar　austenitic,　the　skeletal　and　the　lath　ferrite.　There　is　a　little　ferrite　diffuses　and　grows　to　the　direction　of　the　base　metal　and　no　obvious　coarse　grain　in　heat　affected　zone　(HAZ).　Main　alloy　elements　such　as　Cr,　Ni　are　even　distributed　in　the　weld　and　HAZ.　The　microhardness　of　the　HAZ　is　slightly　higher　than　that　of　weld　metal　and　base　metal.　The　tensile　strength　at　temperature　of　183　K,　213　K,　243　K　and　273　K　temperature　are　86%,　96.7%,　94.5%　and　92.6%　of　base　metal,　respectively,　which　increases　when　temperature　decreases　and　the　maximum　is　up　to　1260　MPa.　Samples　are　bended　by　180°　without　any　cracks　or　other　defects.