Abstract: The 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 the concept of relative root width of weld was proposed as the criterion to evaluate penetration status in CO₂ laser-MIG hybrid welding.Base on relative root width of weld,the penetration statuses including moderate full penetration,partial penetration and excessive penetration were distinguished,and the effects of the variation of welding parameters and gap width on penetration status were further investigated.The results show that the relative root width can be increased by the change of welding parameters,and so the penetration status is changed.Under certain situation,as the gap width increases,the penetration status changes from partial penetration to moderate full penetration,and from moderate full penetration to excess penetration.