YAG laser, TIG arc or their hybrid welding was carried out on Type 304 steel plate under various conditions. The penetration, porosity formation, laser-induced plume, keyhole behavior, bubble formation and melt flows in the molten pool were investigated through light microscopy, X-ray inspection test, high-speed video and X-ray real-time transmission imaging system. It was confirmed that the penetration and geometry of respective welds were affected by welding process and conditions, in particular the shielding atmosphere and the distance between the laser beam and the electrode tip. The weld bead geometry depended upon the oxygen content in the weld metal. The X-ray transmission observation system showed that melt flows in the molten pool were different by the oxygen content and that a pronounced "nail head" disappeared in the atmosphere of higher oxygen content. It was confirmed that the melt flows induced by the surface tension and electromagnetic force exerted a great effect on the penetration and geometry of the weld bead.