Fischer-Tropsch (FT) synthesis with Co catalysts supported on mesoporous silica (SBA-15) with narrow pore size distribution has been carried out with a fixed bed stainless steel reactor at 503 K and 2.0 MPa. When 20 mass % Co is supported on SBA-15 with an average pore diameter of 8.6 nm by using an ethanol solution of Co acetate, nitrate, or an equimolar mixture of these compounds, denoted as Co(20A), Co(20N), or Co(10A+10N), respectively, the Co(20A) is almost inactive in FT synthesis, whereas the Co(20N) and Co(10A+10N) drastically enhance CO conversion, which reaches 85-90%. Such differences arise partly from the formation of less or more reducible Co species. The latter two catalysts show selectivity to C-10-C-20 hydrocarbons of 30-32 C-mol % and provide high space-time yields of this fraction as the main component of diesel fuel, 260-270 g-C/kg-catalyst-h. The yield with the Co(20N) catalyst depends on the amount and has a maximal value of 350 g-C/kg-catalyst(.)h. The N-2 adsorption, X-ray diffraction, and temperature-programmed reduction measurements reveal that pore structures and dispersion states of Co(20N) and Co(10A+10N) catalysts are not changed significantly, even after FT synthesis and subsequent air calcination at 773 K.