Abstract: The computational formulas of CO₂ partial pressure p_CO2 and[C] activity coefficient f_C,% under the condition of high carbon concentration and low temperature were derived from the analysis of limestone decomposition and CO₂ oxidation in the early stage of BOF steelmaking by the thermodynamic method. It is shown that the equilibrium decomposition reaction temperature of calcium carbonate in limestone near the surface of molten iron with high carbon concentration and low temperature is much lower than that under the standard state. The reaction tendency of limestone decomposition increases as the furnace temperature rises,and the reactions of CO₂ with[C],[Si],[Mn]as well as Fe(l) can occur spontaneously as the same reaction sequence of oxidation by O₂ in the early stage. The value of p_CO2 under the condition of high carbon concentration and low temperature is very small,near the value of 0.002 2 to 0.000 5p^Θ in the early stage of BOF steelmaking,so all of the CO₂ produced from the limestone decomposition reaction can be thought to participate in the oxidation reaction of molten iron.