Abstract: This paper deals with a new type square wave power supply for A. C. arc welding in which very fast switching element IGBT is used for its inverter. In the paper, the principle of the power supply and the factors affacting the stability of the square wave A. C. arc are disscussed. And the welding tests show that this power supply can be satisfactorily used for TIG welding of aluminum without any arc stabilizer at very low OCV(30V) and very small welding current (15A).