Catalytic decomposition of methane to pure hydrogen is a reaction of crucial importance for clean energy, if the problem of catalyst separation is solved and the carbon material has an increased commercial value. Unsupported nickel catalysts were synthesized by fusion method. The catalyst derived from nickel nitrate forms heterogeneous octahedral NiO, whereas the nickel hydroxide precursor results in catalyst containing sponge-like NiO with folding lamellar structure of high porosity. The catalysts reactivity test was conducted with a fixed bed system at 1073 K. The catalyst subjected to hydrogen prereduction proved to be inactive. However, the methane prereduction was found to produce some coke to disperse the Ni particles and thus activated the catalyst. It was found that the higher concentration of methane resulted in a better methane conversion, but a higher deactivation rate. Carbon growth models were formulated to explain the formation of different types of carbon over Ni catalyst. (C) 2014 American Institute of Chemical Engineers