SUS301L stainless steel sheet with thickness of 2 mm is welded using CO2 laser-metal inert gas arc (MIG) hybrid welding. The microstructure and phase composition of joint are analyzed in detail. Furthermore, the influence of assembly gap on weld microstructures and performance is investigated. Results indicate that the joint is composed of fine dendritic structure in the weld center and columnar crystals far away from the center zone. And the columnar crystals are found growing perpendicularly to the fusion line. Grain size increases with the distance from the center of weld. Phase composition in the joint is identified as γ austenite and a little δ ferrite. The solidification type of weld is dominated by ferrite-austenite (FA) mode and the transformation from δ to γ is dominated by massive transformation and short range free diffusion. With the increase of assembly gap, residual δ content and tensile strength of weld decrease gradually. The fracture occurs at the area of coarse columnar grain near the fusion line.