Fischer Tropsch (FT) synthesis is an alternative route to produce ultrahigh pure hydrocarbon fuels. The length of hydrocarbon chain produced from FT synthesis taking place over the cobalt supported catalysts is depended upon the reactant and product diffusion as well as the metal particle size of the catalyst. Some works were reported that the pore size diameter of the support can control the metal dispersion. Therefore, the pore size can be considered as an important parameter for supported FT catalysts. This work aims to study the effect of pore size of mesoporous silica on metal particle size of cobalt-based catalysts and FT performance. The synthesized silica with the average pore size diameter of 7.2 nm and the commercial silica with the average diameter of 6.7 nm were used as supports to prepare CS catalyst samples. The cobalt supported on synthesized SiO2 shows larger cobalt oxide particle with lower dispersion compared to the results of the catalyst prepared using the commercial silica. Large cobalt oxide particle of supported catalyst decreases the interaction between metal and support which results in the increasing reducibility of cobalt oxide species and provides large cobalt metal particle after the reduction process. The FT tests were carried out under atmospheric pressure at 220 ᵒC, GHSV of 300 h -1 and H2/CO feed ratio of 2. The enhancement of selectivity towards long chain hydrocarbons (C5 + ) and FT activity were obtained from the catalyst with larger cobalt particle size. The growth of hydrocarbon chain during polymerization can be facilitated by the large surface of active site formed in the large pore size of support. Thus, the pore size diameter of support is an important key for the FT catalytic performance of cobalt catalyst supported on mesoporous silica materials.