Abstract The effect of the addition of different metals (Cu, Rh, Pd, Ir, and Pt) into supported Ni catalysts on the catalytic performance of methane decomposition into pure hydrogen and carbon was examined. The addition of Pd brought about considerable increases in the catalytic life and in the accumulated yields of hydrogen and carbon at the complete deactivation of the catalyst, while modification with the other metals decreased the yields compared to those for Ni/SiO 2. For the Ni catalysts modified with Pd, the appropriate preparation conditions of the catalysts, the effect of catalytic supports, and the optimum reaction temperature on the methane decomposition were examined. The hydrogen yield attained a high value, 16,000 mol H 2 /mol Pd+Ni, when Pd and Ni were of a mole ratio Pd/(Pd+Ni)=0.5 and a total loading Pd+Ni=37 wt% on carbon nanofiber support. This yield is the highest among those reported so far. Methane decomposition over Ni catalysts modified with Pd produced carbon nanofibers with unique structures, i.e., branched carbon nanofibers. XRD studies on the Ni catalysts modified with Pd indicated the formation of Pd–Ni alloys. The alloys are responsible for the increase in the catalytic life and the formation of carbon nanofibers with unique structures.