Mossbauer spectroscopy (MES) has been used to study the characteristics of iron catalysts pretreated in different ways for Fischer-Tropsch (F-T) synthesis. The Mossbauer spectrum results showed that the fine particle of alpha-Fe2O3 is the main phase of oxide catalyst. Catalysts pretreated in H-2:CO ratio of 2.1 syngas at 250 degrees C and under 0.5 MPa with the gas hourly space velocity of 1500 h(-1) for 32 h in a fixed-bed reactor resulted in the reduction of 91% of iron to Fe3O4, FeO and chi-Fe5C2. The chi-Fe5C2 content of reacted catalyst increased greatly as compared with reduced catalyst, while FeO and Fe3O4 contents decreased. The Fe3O4 content of catalyst poisoned by oxygen increased as compared with reduced catalyst. FeS2 is the main phase of catalyst poisoned at high COS concentration, while FeS is the main phase of catalyst poisoned by COS at low concentration. The Fe3O4 content decreased with the increase of sulfur fed to catalyst. The deactivation rate is related to the composition of the catalyst treated in concentrations.