In the present authors' previous study, the rate of iron carbide formation from completely reduced iron ore was gravimetrically measured in CO-H2-H2S mixtures of sulfur activity 0.035 to 0.6 at the total pressure of 0.1 to 0.6 MPa and at the temperature of 773 to 1 023 K. From the experimental results under the wide conditions, it was derived that the rate of iron carbide formation was controlled by the carburization reaction on the pore surface of reduced iron. In this study, the integrated rate equation for the first order reaction, −ln(1−fθ)=g(pi, T)t, was applicable to cementite formation curves, fθ versus t relation, in the range of iron carbide formation, i.e. fθ<0.6. The value of g(pi, T) is proportional to the partial pressure of carbon monoxide, pCO, in the reaction gas containing hydrogen more than 20%. From the dependency of g(pi, T) on pCO, it was assumed that the rate controlling elementary reaction for the carburization of reduced iron is the dissociative adsorption of carbon monoxide on the pore surface. Following the elementary reaction, the rate equation for the carburization was derived substituting the adsorption isotherm for sulfur; g(pi, T)=kCO(pCO−aC pH2O/K2pH2)/(1+KSaS). The relation between pCO/g(pi, T) and aS derived by rearranging the rate equation was applied to the data for g(pi, T) to obtain kCO and KS. At the individual temperatures, the good linear relation was observed. The temperature functions for kCO and KS were obtained. The rate equation substituted with the values of kCO and KS for the carburization simulated reasonably well the carbide formation curves in the range fθ<0.6.