Abstract: This paper studies the measurement method of martensite carbon content of high carbon alloy steel which is quencShed in two phase fields. Through theory analysis a mathmatical relation between martensitic {112} crystal plane diffraction peak is established. In order to simplify the calculation process and make the method more useful in practice, a calculation program is designed. Through measuring some steels whose carbon contents of martinsite are known, the reliability of the method is proved. In the stady of 86Cr3 steel, the curves of relation between quench temperature and holding time with the carbon contents of marfensite are gained by useing this method.