Laser　welding　of　modified　316LN　steel　with　a　thickness　of　20mm　was　conducted　using　a　YLS-20000　fiber　laser.　The　microstructure　of　the　weld　joint　was　characterized　and　tensile　and　fracture　toughness　tests　were　carried　out.　The　microstructure　evolution　of　the　fracture　specimens　was　studied　systematically　to　elucidate　the　fracture　mechanism.　The　weld　was　composed　of　the　single　austenite　phase　and　was　characterized　by　cellular　and　columnar　grains,　and　the　grain　size　became　coarser　with　a　weak　orientation.　The　R-m　values　of　the　laser　welded　joints　were　almost　equal　to　that　of　the　base　metal　both　at　RT　and　4.2　K,　and　the　fracture　surfaces　were　featured　by　ductile　fracture　with　quantities　of　dimples　and　microvoids.　The　fracture　toughness　of　the　weldments　decreased　to　(similar　to)84%　of　that　of　the　parent　metal　at　4.2　K.　Enhancement　of　the　fracture　toughness　was　attributed　predominantly　to　the　presence　of　twins　and　to　the　fine　grain　size,　and　the　reduction　was　caused　by　the　partially　stress-induced　phase　transformation　of　the　austenite　to　martensite.　The　synergistic　effects　of　these　factors　result　in　a　favorable　improvement　in　the　fracture　toughness　of　the　weldment.