Abstract: CO₂ gas was used as the shielding gas to eliminate porosities in deep penetration laser welding of low-carbon steel in this paper.By using a 4 kW Nd:YAG laser,both sides welding of 12mm carbon steel plate was performed under the welding speed of 0.3 and 0.5m/min,using CO₂ and Argon respectively as the shielding gas.The chemical compositions,microstructure and hardness distribution of weld metal were analyzed.The impact toughness of weld metal was evaluated by conducting a kind of non standard three notches impact test,which test piece is apt to fracture within weld metal without deviating into the nearby base metal during impact test.The experimental results show that by using CO₂ as the shielding gas,it is not only possible to reduce or eliminate the porosities in weld metal,but also to reduce the hardness and increase the impact value of laser welds.