Core-shell structured catalysts with cobalt nanoparticles coated by the silica shell were prepared by an in situ coating method to explore the dependence of their Fischer-Tropsch synthesis performances on catalyst structures. The reduction degrees of the catalysts were decreased by silica coating as evidenced by H-2-TPR results. However, the catalytic activity was not decreased due to the enhanced and stabilized particle dispersion by the shell. In addition, such core-shell structures gave rise to a higher selectivity toward methane with a lower proportion of C19+ fraction as compared with an uncoated sample, which had cobalt particles located outside the silica spheres. It was considered that the shell coating not only resulted in severe diffusion restrictions of reactants but also retarded secondary reactions of olefins, both of which shifted product distribution toward the lighter end. As a result, the shell coating improved the selectivity to middle distillates with a higher percentage of olefins. (C) 2013 Elsevier B.V. All rights reserved.