The effect of the active element oxygen and the welding parameters (welding speed, welding current and electrode gap) on the liquid pool convections and the weld shape variations under helium gas shielding is systematically investigated using a mathematical model of the welding arc and weld pool during a moving GTAW of SUS304 stainless steel. Different welding parameters will change the temperature distribution on the pool surface, and affect the strength of the Marangoni convection and the weld shape. The inward convection induced by the electromagnetic force is contributive to the increase of the weld depth. A higher welding speed, lower welding current or larger electrode gap will make the weld D/W ratio decrease when the oxygen content in the weld pool is high. Under low oxygen content, a lower welding speed, welding current or shorter electrode gap will lead to a higher weld D/W ratio. The predicted weld D/W ratio by simulation agrees well with the experimental results. (C) 2010 Elsevier Ltd. All rights reserved.