Based on previous observation that the addition of manganese onto Ni/Al2O3 catalysts reduced the coke formation in the carbon dioxide reforming of methane, effects of the preparation method for Mn-promoted Ni/Al2O3 catalysts on the catalytic activity and stability were investigated. A coprecipitated catalyst, Ni-MnOx/MnAl2O4, showed higher coke resistance and more stable activity than an impregnated Ni/MnO/gamma-Al2O3 catalyst and the well-known Ni0.03Mg0.97O catalyst at 923 K with a feed gas ratio CH4/CO2 of 1 without a diluent gas. In coprecipitated Ni-MnOx/MnAl2O4 catalysts, MnAl2O4, gamma-Al2O3, and Ni(0) were observed as major crystalline phases with XRD and potassium was detected on the surface with EPMA. Comparison of H-2 chemisorption and XRD suggested that the surface of large metallic nickel particles was partly blocked by manganese oxides, and the structure was confirmed by XAFS. For impregnated Ni/MnO/gamma-Al2O3 catalysts, addition of potassium stabilized its catalytic activity further, to the level achieved by coprecipitated Ni-MnOx/MnAl2O4 catalysts. These results indicate that very stable Ni/Al2O3 catalysts for the carbon dioxide reforming of methane can be prepared by addition of both potassium and manganese as promoters.