Detailed phase transformation in syngas (H-2/CO = 1.2) on a precipitated iron-based catalyst was studied by N-2 physisorption, X-ray diffraction (XRD), Mossbauer effect spectroscopy (MES), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy (LRS). Fischer-Tropsch synthesis (FTS) performance of the catalyst was investigated in a slurry-phase continuously stirred tank reactor (STSR). The hematite in the fresh catalyst was reduced initially to magnetite, and then the magnetite in the bulk reached steady state slowly with increasing reduction time. Simultaneously, the Fe3O4 on the surface layers converted gradually to ion carbides, accompanied with the continual increase in amounts of surface carbonaceous species. In the FTS reaction, the catalytic activity presented an increased trend with gradual carburization of the catalyst by keeping the stability in the bulk Fe3O4, suggesting that the conversion of magnetite to iron carbides in the near-surface regions provides probably the active sites for US. In addition, the chain growth reaction was restrained and the hydrogenation reaction was enhanced with increasing reduction duration. (C) 2009 Elsevier B.V. All rights reserved.