Carbon nonotubes filled with long continuous cobalt nanorods or nanoparticles were synthesized by a simple catalytic process. Co(CO)3NO was used as the precursor and served as both the source for the carbon nanotubes and for the cobalt, the catalyst. Some nanotubes filled with nanorods have big heads. The Co-filled nanotubes can be separated easily from the products by using a permanent magnet. The average diameter of the multiwalled nanotubes is about 40 nm. The cobalt nanorods are a few micrometers long with a diameter of 20 nm. In addition, some long, ribbonlike structures of flattened nanotubes were also observed in the product mixture. After sonication of the product mixture, a small number of nanotube rings with cobalt nanoparticles were found. X-ray diffraction analysis shows the cobalt nanorods encapsulated in the carbon nanotubes as having an fcc and not the stable hexagonal structure. A growth mechanism for these filled nanotubes is proposed. XPS and Raman spectrum were also employed to analyze the products. The dc magnetic measurements were conducted to test for its magnetic properties.