The influence of the effect of average pore diameter of silica support on the physical and chemical properties of supported cobalt catalysts and their performance in the Fischer-Tropsch synthesis was investigated. Silicas with different mean pore diameter (20, 40, 60, 100 and 150 Angstrom) were impregnated with cobalt nitrate to produce catalysts containing 20 wt.% cobalt. The metal crystallite size and degree of reduction was found to increase with increasing pore diameter of the support for supports with an average pore diameter larger or equal to 40 Angstrom, and hence the dispersion decreased. In impregnated catalysts, the metal crystallites seems to appear in clusters on the support. With increasing average pore diameter, the size of these clusters increases. In the Fischer-Tropsch synthesis, the 100 Angstrom supported catalyst proved to be the most active and selective catalyst for hydrocarbon formation. The C5+ and methane selectivity passed through a maximum and minimum at the 100 Angstrom supported catalyst, respectively, which can be explained quantitatively using the reactant transport model proposed by Iglesia et al.