Both mono- and bi-modal Co-based Fischer-Tropsch Synthesis (FTS) catalysts were prepared by incipient-wetness impregnation (IWI). XRD and N-2 physisorption revealed that the catalyst with a bi-modal distribution of 2.5-17 nm had the smallest size of cobalt crystal. In this case, the Raman absorbance shifted toward lower frequencies due to the size quantization effect. Furthermore, H-2-TPR indicated a lower reducibility originated from the interaction between small crystalline cobalt and silica. Such bi-modal structure catalysts showed a better FTS performance, and particularly the bi-modal mesopores catalyst presented the lowest methane selectivity, the highest activity and the highest selectivity to C-5-C-18 hydrocarbons, which might be due to the confinement of mesopore to the cobalt particles.