The effect of a Ru promoter on the catalytic properties of the Co/alpha-Al2O3 catalyst in dry reforming of methane (DRM) was investigated. DRM was performed after in situ self-activation by the reactants without a pre-reduction treatment using H-2. The CH4 conversion of the catalyst without Ru promotion gradually decreased during DRM, whereas that of the Ru-promoted one slowly increased. The Ru-promoted Co/alpha-Al2O3 catalyst kept stable conversion of CH4 and CO2 in the sequential reaction consisting of DRM and CO2 treatment. This indicated that the added Ru stabilized the Co sites in the reduced state during DRM. In addition, a small amount of coke was deposited on the Ru-promoted Co/alpha-Al2O3 catalyst compared to the Co/alpha-Al2O3 catalyst after DRM. The enhanced stability of the Ru-promoted Co/alpha-Al2O3 catalyst was ascribed to facile removal of the deposited coke and suppression of metal oxidation. It can be deduced that the Ru promoter provided active sites during DRM, and structural stabilization of Co species.