Magnetite Fe3O4 ultrafine powder, a magnetic material, was synthesized by a novel arc discharge method of metallic iron filaments in NaCl electrolyte. The size and shape of the products were influenced by the voltage between the two metallic iron filaments. Ultrafine iron oxides are important materials for magnetic and pigment applications. As a conventional magnetic material, nanocrystalline Fe3O4 powders with a spinel structure have potential applications for ferrofluid, magnetic refrigeration ....