Fischer-Tropsch synthesis (FTS) for the production of gasoline and middle distillates in the range of C-5-C-22 hydrocarbons was investigated using ZSMS-modified Co/SiO2 FTS catalysts. The Co/SiO2 catalyst was prepared by an impregnation method using a cobalt nitrate precursor in a slurry of SiO2, and theh ZSMS-modified Co/SiO2 catalyst was subsequently prepared by synthesizing ZSM5 (Si/Al ratio = 40) by in situ hydrothermal synthesis over the Co/SiO2 catalyst at different ZSMS/(Co/SiO2) weight ratios. The catalytic performance was largely altered by changing acidity and reducibility of cobalt oxides on ZSM5-modified Co/SiO2 catalysts due to the different extents of migration of cobalt oxides from SiO2 to the ZSMS surface with different degrees of catalytic olefin cracking reaction. The ZSMS-modified Co/SiO2 catalyst with 25 wt % ZSM5 shows a maximum selectivity to C-5-C-22 hydrocarbons with a high CO conversion due to the smaller cobalt crystallite size possessing a high reducibility and optimum acid site density. The superior catalytic performance on the ZSM5-modifiecl Co/SiO2 catalysts is mainly attributed to the modification of the Co/SiO2 surface with the presence of adjacent ZSM5 particles and the possible migration of cobalt species from SiO2 to the ZSM5 surface during the hydrothermal synthesis of ZSMS.