Mesoporous molecular sieves (MCM-48, SBA-15 and SiO2) with different pore diameters have been used as supports of Co catalysts for Fischer-Tropsch (F-T) synthesis. The catalysts were characterized by N-2 physisorption, X-ray diffraction (XRD), temperature programmed reduction (TPR), temperature programmed desorption (H-2 TPD) followed by pulse oxygen reoxidation and transmission electron microscopy (TEM). The catalytic performace for FT synthesis was tested with a fixed bed reactor. TEM and H-2 TPD showed that the cobalt crystallite size formed on Co/SBA-15 with the pore diameter of 5.3 nm were the largest, and the ones formed on Co/MCM-48 with pore diameter of 2.6 nm were the smallest. The Co/SiO2 with the average pore diameter at 10.4 nm is most active for FTS. The Co/SBA-15 presented the highest selectivity to C5+. The activity of the three catalysts was affected by the reducibility of cobalt oxides and C5+ selectivity was determined by the cobalt crystallite size.