Hybrid welding of stainless steels or aluminum alloys was performed using the heat sources of YAG laser and TIG, or YAG laser and MIG, respectively. The effects of welding conditions and melt flows on penetration depth, weld bead geometry and bubble/porosity formation were investigated with X-ray transmission real-time observation method. A great effect of melt flows on penetration depth and weld geometry was consequently confirmed. Concerning porosity suppression in YAG-TIG hybrid welding of stainless steel, no bubble generation was attributed to no porosity formation. On the other hand, it was revealed that the disappearance of bubbles from the concave molten pool surface played an important role of no porosity in YAG laser-MIG hybrid welding of aluminum alloys.