Welding　of　5083-H116　aluminum　plate　with　thickness　of　4　mm　using　IPG　YLS-6000　fiber　laser　is　presented.　The　influence　of　parameters　on　the　weld　forming　and　weld　defects　is　carried　out　in　this　paper.　Also,　microstructure　and　mechanical　properties　of　joint　is　analyzed.　It　indicates　that　laser　power,　speed　and　defocus　have　great　effects　on　penetration　state.　The　main　problems　of　weld　of　5083-H116　aluminum　plate　are　sag　and　undercut.　Under　the　condition　of　penetration,　using　high　power　of　5　kW　and　6　kW　and　speed　range　from　6　m/min　to　9　m/min　can　get　ideal　weld　formation.　With　the　optimal　parameters　of　zero　defocus,　6　kW　power　and　9　m/min　welding　speed,　metallographic　observations　indicate　that　crystalline　morphology　near　the　fusion　line　is　dense　columnar,　and　in　center　of　the　weld　which　is　isometric　dendrite.　Besides,　the　segregation　of　weld　is　more　visible.　The　test　shows　that　the　micro-hardness　of　weld　fluctuates　and　the　mean　value　is　less　than　the　parent　material.　Tensile　strength　of　joint　is　up　to　287　MPa,　about　83.9%　of　parent　material.　Yield　strength　is　about　227　MPa.　Elongation　is　about　3.57%.　The　tensile　fracture　happens　in　weld,　and　belongs　to　ductile　fracture.