With strategical selection of the water-soluble sodium chloride (NaCl) as the supporting material, we realized large-scale production of carbon-encapsulated cobalt nanoparticles with a productivity of almost 100%. The products can be fully separated from the supporting materials by simple washing process. When NaCl, NaF and Al2O3 were used as supporting materials, the morphologies of the carbon products changed from carbon-encapsulated magnetic nanoparticles (CEMNs) to an intermediate state (quasi-nanocages) between CEMNs and carbon nanotubes (CNTs), and then to CNTs, respectively. NaCl shows strong inhibiting effects on the growth of CNTs, favoring the formation of CEMNs. (C) 2002 Published by Elsevier Science B.V.