There is a growing body of experimental evidence that iron carbides can play an important role in the mantle both as a host of carbon and as a redox couple (Fe-C) determining the nature of reduced phases. If carbides are significant in the mantle, it could be of interest to know if any carbon isotope fractionation accompanies carbide crystallization. A series of high-pressure and high-temperature experiments were performed on carbide crystallization from a Fe-C melt. An offset of 2‰ was observed between the δ13C values of Fe3C and Fe-C melt at 6.3GPa and 1400°C. The carbon isotopic compositions of Fe3C and diamond crystallizing from a single carbon source near the peritectic region at 6.3GPa differ by 2.5‰. Fe7C3 was detected as a quench phase during Fe-C melt quenching. Our results have important implications for understanding carbon isotope distributions in iron meteorites and indicate that iron carbide crystallization may be a significant mechanism for carbon isotope heterogeneity in the Earth's mantle.