Autothermal reforming (ATR) of methane was carried out over nanocrystalline Al2O3-supported Ni catalysts with various Ni loadings. Mesoporous nanocrystalline gamma-Al2O3 powder with high specific surface area was prepared by the sol-gel method and employed as support for the nickel catalysts. The prepared samples were characterized by X-ray diffraction, Brunauer-Emmett-Teller, temperature-programmed reduction, temperature-programmed hydrogenation, and scanning electron microscopy techniques. It is demonstrated that the methane conversion increased with increasing in Ni content and that the catalyst with 25 wt% Ni exhibited the highest activity and a stable catalytic performance in the ATR process, with a low degree of carbon formation. Furthermore, the effects of the reaction temperature, the calcination temperature, the steam/CH4 and O-2/CH4 ratios, and the gas hourly space velocity on the catalytic performance of the 25% Ni/Al2O3 catalyst were investigated.