NiO/θ-Al2O3 catalysts were prepared and investigated after reduction treatment in the methane conversion reactions, such as oxy-reforming, steam reforming and oxy-steam reforming. Among the catalysts with various Ni loading, 12% Ni loading exhibits not only the highest catalytic activity and selectivity but also remarkable stability. The TPR results reveal that strong interaction between Ni and support results in forming stable NiOx species. Reducibility of NiO and the population of NiOx compared with NiO and/or NiAl2O4 play very important roles in the catalytic activity and stability of Ni/θ-Al2O3. Metallic Ni sites formed from the reduction of both NiO weakly interacting with the support and NiOx, strongly interacting with the support are active sites for methane reforming reactions. Even though free NiO sites are prerequisite for high activity, the increase of NiO population in high Ni loading favorably promotes Ni sintering resulting in undesirable catalytic properties.