The partial substitution of O₂ with CO₂ in steelmaking process is an important technology to reduce CO₂ emission and recycle. In order to improve CO₂–O₂ decarburization utilization, the decarburization kinetics of Fe–C melt with CO₂–O₂ mixed gas have been studied using ¹³CO₂–¹⁸O₂ dual isotope tracing method and established the decarburization kinetic model. The results show that less than 40% of the oxygen is partially replaced by CO₂, which improves the O₂ utilization involved in the decarburization reaction and the CO₂ utilization is still more than 80%. For the rate limiting steps, regarding O₂, it is governed by the mixed control mechanism involving gas-phase mass transfer and interfacial chemical reaction; regarding CO₂, with the increase of CO₂ partial pressure, the rate limiting step changes from the mixed control to gas-phase mass transfer control only. As bath temperature increase from 1723 to 1873 K, the overall decarburization rate increases; bath temperature mainly affects O₂ decarburization rates, whereas, the rates of CO₂ are not significantly affected. The apparent activation energy of CO₂–O₂ mixed gas decarburization is 21.5 kJ·mol⁻¹.