The paper introduces the results of mixing and fluid flow studies for full penetration laser-gas metal arc (GMA) hybrid welding with GMA leading and trailing torch configuration. The material used was austenitic stainless steel with the thickness of 10mm. The joint configuration was butt joint without gap. It was found that the molten pool at the root side is a longer time in the molten state than the pool at the weld surface. Compared to autogeneous laser welding, the addition of GMA makes the upper bead wider. This effect is more prominent in the case of GMA trailing compared to the case of GMA leading. The simulation results also show a similar trend of bead shape change in the case of GMA leading and trailing configuration compared to the experimental ones. Mixing behavior of filler and base metal is also evaluated and compared by both experimental studies and simulations. In GMA leading configuration, the mixing is more pronounced in the upper part of the weld cross section, while in GMA trailing, the mixing is more efficient in the whole cross section compared to GMA leading case. Simulation results support this difference in mixing behavior. Laser is the main source that provides full penetration and forms a molten pool on upper and root surface. The direction of wire feeding affects the mixing behavior. In GMA leading configuration the wire droplets impinge in front of the laser beam and in the front edge of the molten pool, whereas in GMA trailing configuration, the wire droplets impinge behind the laser beam where the molten pool is fully developed. This causes the difference in mixing. (C) 2017 Laser Institute of America.