Co/SiO₂ Fischer-Tropsch synthesis (FTS) catalysts were prepared by a novel preparation method, where aqueous Co nitrate solutions containing various organic acids and/or chelating agents were used as impregnating solutions. FTS activities were investigated at 503 K and 1.1 MPa. Preparation using some chelating agents improved the FTS activity of Co/SiO₂ catalyst. Especially, the catalyst prepared using nitrilotriacetic acid (NTA) showed the FTS activity ca. 3 times greater than the catalyst without chelating agents. A volcano-type dependency giving a maximum with NTA was found when the FTS activity of these catalysts was plotted against logarithmic complex formation constants with Co ion in the aqueous solution, suggesting that the formation of Co complex with moderate stability (i.e. NTA-Co complex) is crucial for obtaining higher FTS activity. Hydrogen chemisorption measurements on reduced catalysts showed that the preparation of the catalysts using these chelating agents and/or organic acids leads to the formation of fine metallic Co clusters with 1-5 nm size. TOF of these catalysts was independent of the metallic cluster size in 1-17 nm range, which was evidently different from those reported previously. The formation mechanism of fine metallic clusters was also investigated by means of XPS and Co K-edge EXAFS. [DOI: 10.1380/ejssnt.2009.633]