The erosion of tungsten electrode in GTA (Tungsten-inert-Gas Arc) welding under the protection of argon with a little of oxygen contamination have been investigated. Results show that the erosion of tungsten increases greatly if a little of oxygen is add in argon gas because tungsten on the surface of electrode reacts with oxygen to form tungsten oxide. Melting and evaporation of thoria were the major reasons for the erosion of thoria in electrode. To ensure the purity grade and flow rate of shielding inert gas, to use tungsten electrode activated with nanosized thoria are two better ways to decrease the erosion rate of electrode.