Experimental methods measuring arc efficiency of gas tungsten arc welding (GTAW) to a base metal are compared in order to understand heat transfer of GTAW with and without motion of welding torch. Taking the effect of heat loss into account, arc efficiency measurements of GTAW moving at constant velocity are conducted. From a perspective of thermal conduction in a base metal, it is suggested that the temperature gradient inside the base metal affects arc efficiency and that arc efficiency measured by water-cooling copper anode also depends on thermal conduction inside the anode.