The impact of acidity and pore size on the activity and product selectivity of the Fischer-Tropsch (FT) synthesis was comparatively investigated over cobalt supported on a series of alumina grafted siliceous SBA-15 (Al-SBA-15) via the post synthesis method in the medium of supercritical CO2. X-ray diffraction (XRD) results indicate that the ordered mesoporous structure of SBA-15 is preserved after grafting different amounts of alumina. However, both the Brunauer-Emmett-Teller (BET) surface area and the pore volume of Al-SAB-15 are decreased with an increasing content of alumina. Ammonia temperature-programmed desorption (NH3-TPD) results indicate that the acidity of Al-SBA-15, which is dominantly contributed by weak acidic sites, increases continuously with an increasing content of alumina. The 20 wt % Co-supported catalysts prepared by the incipient impregnation method were evaluated for FT synthesis in a fixed bed reactor under the conditions of 1.0 MPa, 235 degrees C, H-2/CO = 2, and W/F = 5.02 g h mol(-1). Significantly, Co/SBA-15 shows the highest CO conversion and the lowest selectivity of long chain FT hydrocarbons (C21+). Moreover, both the activity and product selectivity of FT synthesis over Co/Al-SBA-5 are strongly dependent upon the content of alumina. On the basis of the XRD, transmission electron microscopy (TEM), hydrogen, temperature programmed reduction (H-2-TPR), hydrogen-temperature programmed,desorption (H-2-TPD), O-2 titration, and N-2 adsorption-desorption results, of the catalysts, the reduction behavior and the extent. of reduction of the catalysts are revealed to be main factors for determining the FT activity. Moreover, the observed product Selectivity is reasonably attributed to the varied diffusion limitations of the small pores on the desorbed hydrocarbons and the cracking of long chain FT hydrocarbons over acidic sites, the extent of which are closely related to the Pore structure, location of Co species, and acidic properties of the catalysts.