By hot swaging, the nanocomposite W-4.5%ThO2 cathode was fabricated. A comparative study on the anti-erosion ability for a conventional W-2%ThO2, W-4.5%ThO2 cathode and a nanocomposite W-4.5%ThO2 cathode has been made by using gas-tungsten arc (GTA) welding. The results indicated that the relative density of the nanocomposite material is near full. The microstructure of the nanocomposite cathode is quite different from that of conventional thoriated tungsten cathodes. Most of the thoria particles in the nanocomposite are less than 200 nm in diameter, and situated on the tungsten grain boundaries. The arc starting characteristic of the nanocomposite W-4.5%ThO2 electrode is the best among the three cathodes. The cathode tip shape after arc burning was obseved. The results show that the anti-erosion ability of the nanocomposite W-4.5%ThO2 cathode had been obviously improved, which indicated that the cathode characteristics depend mainly on the content and partcle size of thoria.