The effect of vanadium promotion on activated carbon (AC)-supported cobalt catalysts in Fischer-Tropsch synthesis has been studied by means of XRD, TPR, CO-TPD, H-2-TPSR of chemisorbed CO and F-T reaction. It was found that the CO conversion could be significantly increased from 38.9 to 87.4% when 4 wt.% V was added into Co/AC catalyst. Small amount of vanadium promoter could improve the selectivity toward C-10-C-20 fraction and suppress the formation of light hydrocarbon. The results of CO-TPD and H-2-TPSR of adsorbed CO showed that the addition of vanadium increased the concentration of surface-active carbon species by enhancing CO dissociation and further improved the selectivity of long chain hydrocarbons. However, excess of vanadium increased methane selectivity and decreased C-5(+) selectivity.