Iron based catalysts have been widely used in CO₂ hydrogenation reactions and phase transformation during hydrogenation has been noted. Herein, iron oxide was observed to be in situ carburized into Fe₃C, and Fe₃C was identified as the active and stable phase during the reverse water gas shift (RWGS) reaction. Factors such as the reduction degree of iron oxide and reaction temperatures were revealed to affect the phase transformation from iron oxide to Fe₃C. The feed gas ratio of H₂/CO₂ had a decisive effect on the in situ formation of Fe₃C and an optimized ratio of 2 was crucial to avoid the excessive oxidation or reduction of carbide. Ex situ synthesized Fe₃C via CO carbonization had an inferior catalytic performance compared with the in situ generated Fe₃C due to the unexpected but inescapable carbon deposition.