The influence of cobalt particle size on catalyst performance in Fischer-Tropsch synthesis (FTS) has been investigated using inert carbon nanotube (CNT)-supported catalysts. The catalysts were produced by the core reverse micelle reactions with cobalt particles of various sizes (3-10 nm). It has been shown that particle size is proportional to the water-to-surfactant ratio (3-10) used for the catalyst preparation. Very narrow particle size distributions have been produced by the microemulsion technique and at relatively high loading (Co 10 wt%). Selectivity and activity were found to be dependant on cobalt particle size. The FTS rate increases from 0.36 to 0.44 g HC/g(cat.)/h and the C5+ selectivity increases from 89 to 92.5 wt% with increasing the average cobalt particle size from 2-3 to 9-10 nm, respectively. According to TEM analysis, small Co particles (2-6 nm) are mostly confined inside the CNTs where influence of its electron deficiency in the inside surface has changed the commonly expected catalyst's structure-sensitive results. Finally, the CNT-supported cobalt nanoparticles synthesized by the proposed microemulsion technique increased the CO conversion by 15% compared to those prepared by incipient wetness impregnation. (c) 2009 Elsevier B.V. All rights reserved.