Catalytic performance of Co catalysts supported on different supports (MgO, Al2O3, SiO2, and TiO2) for the formation of carbon nanofibers through methane decomposition was investigated. Catalytic activity and the life of supported Co catalysts for methane decomposition at 773 K strongly depended on the type of catalytic supports; i.e., the catalytic performances of Co/Al2O3 and Co/MgO were superior to those of Co/TiO2 and Co/SiO2. All the Co catalysts produced carbon nanofibers with relatively uniform diameters (10-30 nm) through the reaction at 773 K irrespective of the kind of catalytic supports, although an average crystallite size of Co metal was different for these catalysts. These results implied that Co metal particles with diameters from 10 to 30 nm grew carbon nanofibers from methane preferentially, while ones larger than 30 nm were inactive for the reaction. The structures of carbons formed by methane decomposition over Co/Al2O3 depended on the reaction temperatures. Multiwalled carbon nanotubes (MWCNT) were produced mainly in the temperature range of 873-973 K, where the catalytically active Co metal particles were present at the tip or in the hollow of MWCNT. On the other hand, helically coiled carbon nanotubes and bamboolike carbon nanotubes were formed preferentially at 1073 K, where Co metal particles could be rarely observed in the carbons. These results implied that the growth mechanisms of these carbons by Co metal particles were changed according to the reaction temperatures. Co K-edge XANES and EXAFS suggested that Co species on the supported Co catalysts were always present as Co metal during the methane decomposition, irrespective of the kinds of catalytic supports or the reaction temperatures.