Abstract: Diamond synthesis experiments were carried out in the Fe-Ni-C system under high temperature and high pressure (HTHP). The real carbon source forming diamond was investigated by characterizing the structure and phase of the catalyst and the metal surrounding the carbon films, by analyzing the phase diagram and by thermodynamic calculations. These show that the catalyst and metal are composed mainly of cementite and the catalysts rapidly become supersaturated by absorbing carbon from the graphite and separate out as cementite (Fe₃C or Fe₂C∶Fe) when diamond begins to form a nucleus. According to the phase diagram, the diamond nucleation and growth are accompanied by the decomposition of the cementite. Thermodynamic calculations indicate that the phase transformation free energy of Fe₃C to diamond and γ -Fe is more negative than that of graphite to diamond in the stable diamond growth region. All of these results indicate that the real carbon source for diamond growth in the Fe-Ni-C system under HTHP is cementite, and not graphite.