The reaction mechanism of methane decomposition over silica-supported Ni catalyst (Ni(5 wt%)/SiO2) was investigated by performing kinetic studies of the reactions using isotopic methane (CH4, CD4, and (CH4)-C-13) and hydrogen (H-2 and D-2). An isotope effect between CH4 and CD4 was observed in the decomposition of methane over the Ni/SiO2 catalyst. In the decomposition of a gas mixture of CH4 and CD4 over the Ni/SiO2 catalyst, the H-D exchanged methanes were not formed. These results suggested that the first C-H bond cleavage in CH4 was the rate-determining step in the decomposition of CH4 into carbon and H-2. The idea well explained the reverse isotopic effect between H-2 and D-2 in the hydrogenation of the carbons deposited on the catalyst back into methane. Moreover, the rate-determining step was supported by the result that only CD4 was formed in the early part of the exchange reaction between CH4 and D-2 (CH4 much less than D-2) The mechanism of carbon deposition on the Ni/SiO2 catalyst was examined by performing successive depositions from (CH4)-C-12 and (CH4)-C-13, followed by hydrogenation of the deposited carbon with H-2. The carbon atoms deposited last were found to be hydrogenated to CH4 at first, suggesting that the carbon atoms did not scramble and the amorphous parts in a carbon fiber would be hydrogenated back into CH4 catalyzed by a Ni particle present at the tip of a carbon fiber.