Transitional metals (M) were dispersed on single. wall carbon nanohorns (M/SWCNHs, M = Fe, Co, Ni, Cu) by simple thermal treatment of the deposited metal nitrate without H-2 reduction. Nanometallic Ni particles on SWCNH were evidenced by high-resolution transmission electron microscopic observation and X-ray photoelectron spectroscopy. The nano-Ni dispersed on SWCNH showed the highest CH4 decomposition activity; the activity of used transitional metals decreases in the order Ni >> Co > Fe >> Cu. On the other hand, the reaction rate over Ni/SWCNH was much larger than that over Ni/Al2O3, and the former provided COx-free H-2 and cup-stacked carbon nanotubes, while Ni/Al2O3 produced COx in addition to H-2. SWCNH was superior to Al2O3 as the catalyst support of Ni for the CH4 decomposition reaction.