A cobalt-silica hybrid nanocatalyst bearing small cobalt particles of diameter ~5 nm was prepared through a hydrothermal reaction and hydrogen reduction. The resulting material showed very high CO conversion (> 82%) and high hydrocarbon productivity (~1.0 g_HC·g_cat^-1·h^-1) with high activity (~8.5 × 10^-5 mol_CO·g_Co^-1·s^-1) in the Fischer-Tropsch synthesis reaction.