Abstract: Co-based catalyst supported on bi-modal MCM-41 molecular sieves was prepared by incipient-wetness impregnation method; the influence of reduction and oxidation pretreatment on the catalyst structure and its performance in Fischer-Tropsch synthesis (FTS) was investigated. XRD results showed that the cobalt species in the catalyst after the pretreatment is in the form of metal cobalt with face centered cubic crystal phase and the crystallite size is increased from 8.4nm to 22.6nm, compared with the untreated one; due to the increase of crystallite size, the Raman characteristic peak shows a blue shift. SEM and TEM results indicated that the cobalt species exhibits good dispersibility. H₂-TPR results indicated that the reduction temperature is reduced and the interaction between metal and support is strengthened after the pretreatment. As a result, the pretreated-catalyst exhibits lower FTS activity and higher selectivity to methane than the fresh catalyst; however, the selectivity to C_5~18 products over the pretreated-catalyst is improved obviously, especially, the selectivity to C_5~11, which is about twice as that over the untreated one.