Abstract: Shielding gas is acrucial factor for the process stability, weld penetration and joint quality of CO₂ laser-MAG (mental active gas)hybrid welding.However, the concerned researches about this field are very few.A serial trial investigating the effect of He -Ar and CO₂ -Ar shielding gas on CO₂ laser -MAG hybrid welding was carried out on mild steel.The results show that different mixed shielding gases have different effects.The penetration depth and microhardness of He -Ar welds are hisher than that of CO₂ -Ar welds.Because atomic oxygen is decomposed from CO₂ under high temperature and enters into welding pool, the surface tension coefficient changes and the direction of weld pool flow is changed.Consequently, CO₂ -Ar weld reinforcement becomes flatter at CO₂ ≥ 30% and the transition from arc zone to laser zone is flatter.Moreover, when CO₂ >30%, the process stability and microhardness of weld dramatically decrease.