A nozzle which consists of two co axis pipes was designed to weld stainless steel with a 2.5 kW CO 2 laser. The influences of the supplying way and flow of argon and helium, and of the angle of nozzle on the control of laser induced plasma were studied. When the angle of the nozzle to the workpiece is greater than 45° with helium flowing through the outer pipe and argon through the inner one, the laser induced plasma is completely suppressed in the keyhole, and then a weld is obtained of great weld depth and great ratio of depth to width. The required flow of helium to control the plasma decreases with the increase of welding speed.