Formation of weld root is an important criterion to evaluate the gap bridging ability and weld quality of hybrid welding in gapped butt welding. Therefore in CO2 laser-MIG hybrid welding of stainless steel with 3 mm thick, the effect of welding parameters such as laser power, arc current, distance of laser to arc, welding speed and gap width influencing root width of weld were investigated. Both laser plasma and arc plasma were also observed by CCD camera. The result indicates that there are four types of penetration status with the variation of welding parameters in CO2 laser-MIG hybrid welding. Fine root formation with “moderate full penetration” status can be acquired with appropriate laser power, arc current, distance of laser to arc and welding speed in certain gap. “Partial penetration” and “unstable penetration” will be produced with smaller laser power, or smaller arc current, quicker welding speed, contrarily it will be “excessive penetration”. Effect of laser power on penetration is not effective for bigger gap width. Effect of distance of laser to arc on plasma and root width of weld has also been discussed.