Cobalt-based catalysts showing high Fischer-Tropsch reaction activity in slurry-phase semi-batch reactor are studied. CO conversion increases with increasing cobalt loading, and chain growth probability increases as well. It is found that catalytic activity shows a linear relationship with the surface cobalt amount. At high cobalt loadings, two kinds or cobalt metal crystallites, cubic and hexagonal, exist, the cubic crystallite being predominant. The effects of reaction conditions, catalyst pore sizes, calcination and reduction temperatures are studied.