Measurements have been made of the rate of Fe₃C formation from Fe₃O₄ powder and Fe₃O₄ single crystals at 773 K and 1023 K under the high pressure of 0.5 and 1.0 MPa by using thermo-gravimetric method. Along with the kinetic study of Fe₃C formation, high resolution TEM observation around the interface between Fe₃O₄ matrix and the formed Fe₃C have been carried out to confirm the thermodynamically possible direct Fe₃C formation from Fe₃O₄ without the formation of intermediate metallic Fe. Analysis of the rate results suggests that the reaction mechanism of Fe₃C formation was possibly described by the sequential reactions of the conversion of Fe₃O₄ to Fe₃C via intermediate metallic Fe although the metallic Fe existence was not confirmed by TEM observation. This inconsistency was explained by considering the relative reaction rates of Fe₃C formation from Fe and Fe formation from Fe₃O₄. It was also found that not only the carbon activity of more than unity but also the low oxygen potential enough for metallic Fe existence might be required for the Fe₃C formation.