Cobalt-filled carbon nanotube composites (see Figure) with a high metal-filling ratio can be prepared by a simple and effective method. The length, diameter, and structure of the metal-filled nanotubes and the wall thickness of the plain carbon nanotubes can be easily controlled by the template used and synthesis conditions. Magnetic measurements reveal that the array of cobalt-filled carbon nanotubes exhibits enhanced coercivities in comparision to that of bulk Co.