Abstract: Microstructures and mechanical properties of welded joint of 5083 aluminum alloy plate with 5 mm in thickness by plasma-MIG plant welding were studied. The results showed that equiaxed grains distribute in weld seam zone, and their sizes distribute uniformly. Fine equiaxed grains appear in the fusion boundary zone and weld center. The grain sizes on both sides of the weld and near the upper and lower surfaces are larger. The width of PMZ (Part of the melting zone) is larger. Micro-hardness value in the weld seam zone and the heat affected zone is less than that of the base metal, and the minimum hardness value of the weld is 78 HV, which is about 85% of that of the base metal. The tensile strength of the test plate is about 277 MPa, and the elongation is 7.34%. The fracture is located near the fusion line.