Methane decomposition into hydrogen and carbon nanofibers was carried out over Ni/SiO2. The catalytic activity of Ni/SiO2 decreased with time on stream and finally the catalyst was deactivated completely. The initial catalytic activity and the yields of hydrogen and carbon nanofibers until complete deactivation of the catalysts depended strongly on the loading amount of Ni. The yield of carbon nanofibers reached the maximum (491 g(C)/g(Ni)) for Ni (40 wt%)/SiO2, which is one of the highest values among those for catalysts reported so far. SEM images of the catalysts after methane decomposition suggested that the particle size of Ni metal controlled the yields of hydrogen and carbon nanofibers. The yields of hydrogen and carbon nanofibers also depended significantly on the reaction temperatures; i.e., the yields decreased sharply with a rise in the reaction temperatures at > 773 K. In addition, the reaction temperatures controlled the diameter and graphitic order of carbon nanofibers. (C) 2003 Elsevier Science (USA). All rights reserved.