Mosshauer spectroscopy (MES) has been used to study the characteristics of iron catalysLs preireated in different ways for Fischer-Tropsch (F-T) synthesis. The Mosshauer spectrum results showed that the fine particle of α-Fe2o3is the main phase of oxide catalyst. Catalysts preireated in H2:CO ratio of 2.1 syngas at 250°C and under 0.5 MPa with the gas hourly space velocity of 1500 h−for 32 h in a fixed-bed reactor resulted in the reduction of 91% of iron to Fe3o4, FeO and X-FC5C2- The κ-Fe5s2content of reacted catalyst increased greatly as compared with reduced catalyst, while FeO and Fe3o4contents decreased. The Fe3O4content of catalyst poisoned by oxygen increased as compared with reduced catalyst. FeS2is the main phase of catalyst poisoned at high COS concentration, while FeS is the main phase of catalyst poisoned by COS al low concentration. The Fe3o4content decreased with the increase of sulfur fed to catalyst. The deactivation rate is related to the composition of the catalyst treated in concentrations.