The vibration modes of hydrogen-atom adsorption on the diamond surface were studied with high resolution-electron energy loss spectroscopy. Two main losses were observed at 360 meV and 160 meV. They are assigned to the C-H stretch vibrations and the angle-changing deformation vibrations respectively. Replacing H-atom with D-atom, isotropic shifts were observed in the loss spectra. Heating the diamond surface to 900℃, all of the loss features disappear, but an inelastic continuous loss-structure was observed. Due to H-atom desorp-tion, the dangling-bonds became horizontal bonds on the diamond surface, resulting in the gra-phitization of diamond surface. The appearance of the inelastic continuous loss-structure was the characteristic of graphite π-band. The graphitization of the diamond surface was affirmed further by UPS and AES studies.