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We studied the effect of crystallite size of nanocrystalline TiO2 derived from the sol-gel method on the properties of Co/TiO2 catalysts. First, nanocrystalline TiO2 supports having different crystallite sizes were prepared via the sol-gel method by varying the water:alkoxide ratios from 4 to 165. Then, they were used as support for the cobalt catalyst. After calcination, the crystallite size of Co3O4 was dependent on the size of TiO2 support. CO hydrogenation under methanation was performed to determine the activity and selectivity of Co/TiO2 catalysts. It revealed that the catalyst with smaller size of TiO2 crystallite exhibited higher catalytic activity due to higher Co dispersion without significant change in product selectivity. In addition, the Ru modification on the TiO2 support was also investigated. The Ru modification resulted in increased activity regardless of the size of TiO2 crystallite.