CeO₂, ZrO₂, and CeO₂–ZrO₂ supported on MgO-promoted Ni catalysts are developed and applied to the reforming of methane with carbon dioxide. The catalysts are prepared by the one-step co-precipitation/aging method and characterized through various techniques such as X-ray diffraction, Brunauer–Emmet–Teller measurements, pore distribution, X-ray photoelectron spectroscopy, X-ray absorption near edge spectroscopy, extended X-ray absorption fine structure analysis, H₂-temperature programmed reduction, H₂-chemisorption, Raman analysis, thermogravimetry analysis, and pulse H₂–CO₂ reactions. Ni–MgO–CeO₂ shows the smallest Ni particle size and the particle size decreases with increasing ZrO₂ content. Ni–MgO–Ce_0.6Zr_0.4O₂ exhibits the largest oxygen storage capacity among the prepared catalysts. The size of the Ni particles and the oxygen storage capacity are found to be the primary and secondary key factors that influence the catalytic performance, respectively. The turnover frequency is dependent on the size of the Ni particles, but the catalytic performance is affected by the number of Ni active sites, which is estimated from the reduction degree and Ni particle size. Overall, the Ni–MgO–Ce_0.8Zr_0.2O₂ catalyst shows the best performance owing to the high reduction degree and small Ni particle size.