Impacts of H-2/CO ratios on both the bulky and surface compositions of an iron-manganese based catalyst were investigated by XRD, MES, N-2-physisorption, XPS and LRS. Fischer-Tropsch (F-T) synthesis performances were studied in a slurry-phase continuously stirred tank reactor. The characterization results showed that the fresh catalyst was comprised of the hematite, which was converted firstly to Fe3O4, and then carburized to iron carbides in both the bulk and surface regions under different H-2/CO ratios atmosphere. Pretreatment in lower H-2/CO ratio facilitated the formation of iron carbides on the surface of magnetite and surface carbonaceous species. During the F-T synthesis reaction, the catalyst reduced in lower H-2/CO ratio presented higher catalytic activity, which is assigned probably to the formation of more iron carbides (especially for chi-Fe5C2) on the surface of magnetite. The increase of CO partial pressure promoted the product distribution shifting towards heavy hydrocarbons. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.