Suppression of CO2 discharged from iron and steelmaking companies is an example of the biggest issues for the protection of global environment and sustainable growth of steelmaking industry. One of the efforts made to decrease the emission of CO2 in ironmaking process is blowing of hydrogen gas into blast furnace. Hydrogen gas can reduce iron oxide and form harmless H2O. Cementite (Fe3C) may be formed by introduction of hydrogen into blast furnace and play an important role on carburization and smelting behavior of reduced iron.In the present work, Fe3C sample was held at 800-1 100 K under Ar atmosphere to clarify the stability and the behavior of Fe3C phase. It was confirmed that metastable Fe3C phase will decompose under Ar atmosphere at 800 K and rapidly decompose at temperature over 900 K. Also, it was found that composite of nano-size C and Fe will form when Fe3C decompose.