A cobalt-silica hybrid nanocatalyst bearing small cobalt particles of diameter similar to 5 nm was prepared through a hydrothermal reaction and hydrogen reduction. The resulting material showed very high CO conversion (> 82%) and high hydrocarbon productivity (similar to 1.0 g(HC)center dot g(-1) (cat)center dot h(-1)) with high activity (similar to 8.5 x 10(-5) mol(CO)center dot g (Co) (-1) center dot s(-1)) in the Fischer-Tropsch synthesis reaction.