A series of cobalt catalysts supported on silica with different pore size were prepared by incipient wetness impregnation method. N-2 physisorption, XP-D, H-2-TPR, H-2-TPD, DRIFTS and O-2, pulse reoxidation were used to characterize the catalysts. The results showed that the pore size of the support had a significant influence on the CO3O4 Crystallite diameter, catalyst reducibility and Fischer-Tropsch activity. The larger pore could cause the CO/SiO2 to form larger CO3O4 crystallite and decreased its dispersion. Catalysts with different pore size showed different CO adsorption property. The catalysts with pore size of 6-10 nm displayed higher Fischer-Tropsch activity and higher C5+ selectivity, due to the moderate particle size and moderate CO adsorption on the catalysts. (c) 2005 Elsevier B.V. All rights reserved.