Carbon nanotubes were synthesized by chemical vapor deposition and subsequently purified and oxidized by repeated treatment with nitric acid. After acid treatment the walls of carbon nanotubes became thinner and the surface area increased. The Ni-supported carbon nanotubes prepared by impregnation were applied to the cracking of n-heptane to show an excellent activity. This indicates that the carbon nanotubes can serve as a good catalyst support with high dispersion of metallic components. The optimum Ni loading turned out to be about 5.2 mmol/g. The cracking product was found to contain mainly small hydrocarbons; thus the Ni-supported carbon nanotubes may be applied as a potential catalyst to the cracking of heavy hydrocarbons.