A series of metal-promoted 20 wt%Co-0.5 wt%Ir-SiO₂ catalysts was prepared by the alkoxide method, and the effects of additives (MO_x; M = K, Cr, Al, Ce, La, and Mn) on the catalysis of Fischer-Tropsch reaction were investigated. K-Promoter reduced CO conversion. Al- or Cr-promoter resulted in almost the same CO conversion as over the catalyst without additives, but CH₄ selectivity increased. La- or Ce-promoter decreased C₅₊ selectivity. In contrast, CO conversion and the α-value (the chain growth probability of CH_x intermediates) increased up to 57% and 0.85, respectively, over catalysts with 10 wt% Mn promoter. However, further addition of Mn caused deactivation of the catalyst. Selectivity for CH₄ showed a linear relationship with the standard enthalpy of formation (-ΔH_f⁰) of the promoter oxides (MO_x).