Fe3C nanoparticles encapsulated in carbon shell with a size range of 20-50 nm were obtained in large scale by reacting anhydrous FeCl3, hexamethylenetetramine and metal Na in an autoclave at 650 degrees C. Magnetization measurements show that the as-obtained materials display superparamagnetic properties at room temperature. A possible formation mechanism of the core-shell nano-structures was discussed. (C) 2007 Published by Elsevier Ltd.