Nanoparticle size effects on catalyst activity and products selectivity of CNT supported cobalt catalysts in Fischer-Tropsch synthesis are studied. CNT supported cobalt catalysts prepared by two different methods (microemulsion and impregnation) are employed to yield higher hydrocarbons from synthesis gas. To achieve a series of catalysts with different Co particle size, water-to-surfactant molar ratio varied from 4 to 12. The results showed that the average particle sizes depended linearly upon the respective ratio and with decreasing the water content in microemulsion method, prepared cobalt particles to control a narrow nanoparticle size distribution with different water to surfactant ratios. The results show that cobalt-time yields increased substantially (from 3.5 to 2.9 x 10(-3) mol CO/(mol(Co)s)), the site-time yields were decreased (from 26.9 to 37.0 x 10(-3) s(-1)), C5+ selectivity and chain growth probability for hydrocarbons decreased when metal particle size reduced.