The 100 mm thickness SUS304 stainless steels are welded by the ultra-narrow gap laser welding, and the microstructures and mechanical properties of the joints are analyzed. The results show that the hundred-millimeter-grade thickness plates can be effectively welded by the ultra-narrow gap laser welding and the welds present good surface appearances. The microstructures of the joints are composed of austenite and small amount of ferrite, and the grains are with the cellular equiaxed shapes. The tensile strength of the joints is 658 MPa and the tensile fracture is located at the cellular grain zone near the fusion line. The fracture morphology shows fine equiaxed dimples and tearing ridges, which indicates that the fracture is ductile. The microhardness at the equiaxed grain zone is obviously higher than that at the cellular grain zone.