Methane decomposition over Ni loaded activated carbon (AC) was investigated in a fixed-bed reactor, and the results were compared with those of the individual methane decomposition over AC. XRD results show that there is no NiO observed, and only Ni metal crystallite is found in the catalyst even if it is calcined in Ar, which eliminates the inevitable reduction step with other supports. When Ni is loaded on AC, the NUAC catalyst shows higher activity in methane decomposition than the original carbon. Ni crystal size increasing and the new crystallite Ni3C formation during the process lead to the deactivation of the catalysts. Filamentous carbon formation is observed and interlaced with the deactivated catalyst surface at moderate condition with low amount of Ni loaded. Temperature has great effect on the catalytic performance of Ni/AC catalyst and the formation as well as the characterization of the filamentous carbons. (c) 2006 International Association for Hydrogen Energy.