It is known that the molten iron oxide reduction by Fe–C melt shows quite different behavior depending on whether the surface of Fe–C melts is fully covered with molten iron oxide or not. There have been only two studies of the molten iron oxide reduction by Fe–C melt with the existence of free surface. In both studies, the reduction appears to be controlled by the reaction of C+O→CO (g). The rate of this reaction has not been fully established. In the present study, based on the reexamination of overall reaction rates of previous two studies by Dancy and Lloyd et al., the forward and backward reaction rate constants of the reaction C+O→CO (g) at 1873 K have been evaluated by applying the order of magnitude evaluation method. The forward rate constant, in the unit of mol/m² s, is given by: k=1.33×10⁸ exp(−250000/RT), and the backward rate constant, in the unit of mol/m² s, is deduced to: k_CO=2.4×10¹¹ exp(−277000/RT).