A series of Co catalysts supported on carbon nanotubes (CNTs) and carbon spheres (CSs) with different cobalt particle sizes (3-45 nm) were prepared by different methods and using different cobalt precursors. The Co/CNTs and Co/CSs can be autoreduced by the supports in N-2 at ca. 480 degrees C; they show better Fischer-Tropsch performance than those reduced in H-2 when the reduction T > 400 degrees C. The turnover frequency (TOF) value for both Co/CNT and Co/CS was constant for cobalt particles above 10 nm and decreased sharply for the cobalt catalysts with smaller cobalt particles. Remarkably, the TOF for 11 catalysts prepared using different precursors and preparation methods on two different carbon supports depends only on particle size. Finally, a positive relationship was observed between cobalt particle size and C5+ selectivity for both Co/CNT and Co/CS catalysts. (C) 2010 Elsevier Inc. All rights reserved.