To clarify the graphene formation process on a diamond C(111) surface, changes in the chemical bonding state caused by annealing in vacuum were investigated by photoelectron spectroscopy using synchrotron radiation. It was difficult to study the formation of sp2-bonded carbon atoms on a diamond C(111) surface using photoelectron spectroscopy because the peak of the sp2 component overlaps the peak of the surface sp2 component as a result of the 2 × 1 reconstruction. Therefore, we focused on the shift in the C 1s photoelectron spectra and energy loss spectra caused by band bending depending on the temperature. As a result, we found that graphitization on the diamond C(111) surface began at approximately 1120 K, which was lower than that for a SiC substrate. The obtained photoelectron spectra indicated that a buffer layer composed of sp2-bonded carbon atoms existed at the interface between graphene and the diamond C(111) surface.