Abstract: This paper makes a advanced theoretical investigation of the behaviour of TIG arc in narrow gap and puts forward a mathematical model of electric field distribution in arc column with experimental verification. Research results point out that the welding arc, being limited by the side walls of the gap, is not in a free burning condition. Hence the electric field distribution is modified and a certain amount of current flows into the side walls. However, the current density in the corners made by the bottom and the side walls is rather small, and it approaches zero when the corners are sharp-pointed. It follows that in narrow gap TIG welding the fusion of side walls could be out of order if no proper technological measures are taken.