Several types of SBA-15 with pore diameters of 3.6-12 nm have been synthesized under different reaction conditions, followed by air calcination, and utilized as supports for loading 10-20 mass% Co catalysts. Post-synthesis heat treatment after reaction at 308 K is more effective for increasing the pore diameter and volume of SBA-15 than the addition of trimethylbenzene before reaction. The combination of the two results in the formation of SBA-15 with large pore diameter but broad size distribution and less organized structures. When an acetone, ethanol, or water solution of Co(NO3)(2) is used as a precursor for impregnation with SBA-15 with pore diameter of 3.6 nm, the average crystalline size of Co3O4 observed at 10 mass% Co exceeds the pore diameter irrespective of the kind of the solvent. On the other hand, the use of an ethanol solution of Co(CH3COO)(2) or an acetone solution of (CH3COCHCOCH3)(2)CO provides no X-ray diffraction peaks of Co3O4 even at loading as high as 20 mass% Co, and catalyst addition considerably decreases the pore volume of SBA-15 support regardless Of the pore diameter. These observations strongly suggest that Co catalysts exist as nanosized clusters inside the mesopores. The SAXS measurements show that the hexagonal mesoscopic organization of the SBA-15 support with pore diameter of 5.5 nm is almost retained even after addition of 20 mass% Co. The 20 mass% Co/SBA catalysts are also tested preliminarily in Fischer-Tropsch synthesis under 503-523 K and 2.0 MPa.