Abstract: Different sizes of γ-Fe₂O₃ nanoparticles (4−19 nm) were prepared by thermal decomposition of iron oleate and carburized in three different gas atmosphere of 5%CO/He, 5%CO/10%H₂/He and 5%CO/20%H₂/He at 350 ℃. The carburization process and phase transformation of γ-Fe₂O₃ nanoparticles were investigated by in situ XRD, Raman spectroscopy, CO-TPR and TEM. The results showed that χ-Fe₅C₂ and θ-Fe₃C phases with a stable ratio were formed after carburization. The time to complete carburization was shortened for increasing sizes of γ-Fe₂O₃ particles under the same carburization atmosphere. While the smaller γ-Fe₂O₃ particles showed more residual carbon on the surface, which could inhibit the carburization process. The relative content of θ-Fe₃C increased with the increase of the size of γ-Fe₂O₃ nanoparticles. For γ-Fe₂O₃ nanoparticles with the same sizes, the time to complete carburization in different atmospheres was firstly shortened and then slightly lengthened with the increase of H₂ partial pressure, while the relative content of θ-Fe₃C increased with the increase of H₂ partial pressure. By adjusting the particle size of γ-Fe₂O₃ and the carburization atmosphere, the mixed phases of χ-Fe₅C₂ and θ-Fe₃C can be optimized.