Abstract Alumina supported cobalt catalysts promoted by Ru were prepared by co-impregnation method. The effect of Ru on the CO hydrogenation capability of Co/γ-Al 2O 3 catalysts was investigated in a continuous strirred tank reactor (CSTR). The catalysts have characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), temperature programmed reduction (TPR), H 2-chemisorption and BET methods and also the results are reported. The selected Ru loadings were 0.5, 1.0, 1.5, 2.0 wt%, while the loading of Co was 20 wt% in all prepared samples. Characterization studies of catalysts promoted with 0.5% and 1.0% Ru showed higher extent of reduction, as well as, better dispersion of Co on the support, reduction of particle size of the metal, and of pore volume of catalysts. These catalysts also exhibited higher CO conversions. By further increasing of the Ru content (1.5, 2 wt%), however, the results were reversed, i.e., the extents of reduction of Co and CO conversion were decreased. However, the C 5+ selectivity of the products was remained rather unaffected. Meanwhile, the catalysts showed higher selectivity on C 5–C 12 hydrocarbons.