Abstract: In the Fischer-Tropsch synthesis reaction, Fe-based catalysts are widely used in large-scale indirect coal liquefaction industry due to their low price, high activity, and low CH₄ selectivity. The catalytic performance is closely related to the catalyst particle size, surface structure and composition. Since reductive carbonization is a key step in the activation of iron-based catalysts, in this work, Fe₃O₄-O (expose the 111 crystal planes) with different particle size, and similar particle size but exposing different crystal planes, 111 and 110 (Fe₃O₄-RD), have been prepared to explore the effect of particle size and surface structure on the carbonization process. The results show that the 50 nm Fe₃O₄-O particles change more significantly than the one with large particle size (2–10 μm) after carbonization. In-situ XRD was used to monitor the phase change of Fe₃O₄ with exposing different surface planes during carbonization. The results show that 150 nm Fe₃O₄-O and Fe₃O₄-RD particles behave differently in carbonization rate and have different iron carbide concentration in the end, which indicates the carbonization process can be affected by exposed crystal planes. TEM analysis reveals that Fe₃O₄@Fe_xC core-shell structure formed after carbonization.