The deactivation mechanism of Co/MgO catalyst for the reforming of methane with carbon dioxide was investigated. The conversion of CH4 displayed a significant decrease in the initial stage caused by carbon deposition. There were two types of cokes, carbon nanotubes (CNTs) and carbon nano-onions (CNOs). The number of the CNO layers that coated on the surface of Co nanoparticles (NPs) increased rapidly in the initial reforming time, which was responsible for the deactivation of the Co/MgO catalyst. The deposition of CNOs was attributed to the oxidation of Co NPs. Therefore, the deactivation of the Co/MgO catalyst was originated from the first oxidization of the Co NPs into Co3O4 by O species (OH intermediate, CO2, H2O) during the reforming reaction, which accelerates the formation of coke that blocked the active metal, thus led to catalyst deactivation. (C) 2018 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.