The addition of a small amount of Al2O3 to silica-supported cobalt catalysts significantly increased the dispersion of cobalt and Co-metallic surface area, resulting in the remarkable enhancement of the Fischer-Tropsch synthesis (FTS) activity in the slurry-phase reaction. The addition of Al2O3 adjusted the interaction between cobalt and the silica support quite well, realizing the favored dispersion and reduction degree of supported cobalt and leading to high catalytic activity in FTS. The properties of various catalysts were characterized by in situ DRIFT, XRD, TPR, N-2 physisorption, and H-2 chemisorption.