The influences of cobalt particle size on the Fischer-Tropsch reaction have been studied by using pre-synthesized cobalt oxide nanoparticles. Cobalt oxide nanoparticles in the range of 3-16 nm were synthesized by reacting colloidal solution of Co3O4 at the different reaction temperatures from 150 to 250 degrees C and stabilized by capping agent. The nanoparticles were impregnated on gamma-Al2O3 to prepare 5 wt% Co/Al2O3 catalysts and compared with the catalysts prepared by conventional methods. The reduction degree by TPR and O-2-uptake increased with the increase in the cobalt crystal size from 4.8 to 17.5 nm, however H-2-uptake decreased. The rapid growth in the catalytic activity was observed when the cobalt crystal size increased from 4.8 to 9.3 nm and then the activity decreased with further increase in crystal size showing volcano shape in catalytic activity. The catalyst with the cobalt particle size of 9.3 nm showed higher CO conversion, better C5+ selectivity and lower methane conversion. These size-defined cobalt catalysts showed considerably higher FTS activity and better TOF(S-1) than conventional catalysts and with the increase in the cobalt particle size, their deactivation rates also decreased as well even after 54 h on stream. (C) 2011 Elsevier B.V. All rights reserved.