In this present study, CO hydrogenation of carbon monoxide via modified titania-supported cobalt (Co) catalysts was investigated. The nanocrystallite size of titania support was used with zirconia modification. Details of this research are as follows; In this present study, the zirconia-modified TiO₂ having different crystallite sizes of 11, 13 and 16 nm as support for the cobalt catalysts were investigated. The different TiO₂ crystallites derived from the sol-gel method was obtained by varying the water:alkoxide molar ratios from 165 to 4. Then, the TiO₂ supports were used to yield various Co/TiO₂ catalysts. It was found that the sizes of cobalt oxides species dispersed on TiO2 were corresponding to TiO₂ sizes. Based on XPS study, the binding energy of Co 2p[subscript 3/2] was not affected by the size of TiO₂. The catalytic activity was measured via CO hydrogenation under methanation condition. In case of unmodified TiO2 support, the smaller TiO₂ crystallites (Co/ TiO₂_11 nm) exhibited higher activity than other larger ones. Moreover, the zirconia modification also resulted in increased activity for all samples. For both cases, it can be attributed to the increased cobalt dispersion with small crystallite size of TiO₂ support coupled with the zirconia modification as measured by CO chemisorption.