The influence of cobalt particle size on catalyst performance in Fischer-Tropsch synthesis (FTS) has been investigated using functionalized carbon nanotube (CNT)-supported nano catalyst. The catalysts were synthesized by wet impregnation and special sol-gel technique. The catalysts were characterized by BET, XRD, H2 chemisorption, TPR, and TEM. According to TEM analysis, small Co particles (3-8 nm) synthesized by sol-gel technique have very narrow particle size distributions and are mostly confined inside the CNT. The deposition of cobalt nanoparticles synthesized by sol-gel technique on the functionalized CNT shifted the reduction peaks to a low temperature, indicating higher reducibility for uniform cobalt particles. The proposed sol-gel technique increased the FTS rate from 0.62 to 0.71 g HC/gcat./h, C5+ selectivity increased 7% and CH4 selectivity decreased 4%, compared to that prepared by incipient wetness impregnation. This new catalyst preparation method may offer an attractive alternative for nanoparticles synthesis with uniform, and various size distributions.