Abstract: A two dimensional compressible gas flow model of laser penetretion welding process was founded for a better understanding of the influence of side gas flow conditions on the behavior of laser induced plasma. The distributions of temperature and velocity within plasma plume under different side gas flow conditions were calculated.The shape and size of plasma in laser welding process was studied with high-speed CCD carnera.The result shows that laser induced plasma and weld geometry are sensitive to side nozzle height, its inclination angle and side gas flow rate; that cooling effect and assist-gas/plasma-plume momentum ration play an important role in controlling the plasma plume.