We have investigated the initial oxidation process at oxide thickness below 3 ML. High-resolution electron energy loss spectroscopy has been used to clarify the local bonding structures of Si-O-Si bonds on H-terminated Si(111)-1 x 1 surfaces and clean Si(111)-7 x 7 surfaces. We observed shifts of the vibrational energy of Si-O-Si asymmetric bending (nu(O1)) smd asymmetric stretching modes with increasing the number of adsorbed O atoms. Furthermore e, a different behavior of the energy shifts of nu(O1) was clearly observed between the H-terminated 1 x 1 and the clean 7 X 7 surfaces. Based on a central-force-network model, it was found that the difference of the structural relaxation in the initial oxidation process between the H-terminated Si 1 x 1 and the clean Si 7 X 7 surfaces appears conspicuously in the bond angle of Si-OSi. This phenomenon indicates that the Si-OSi structure is relaxed by making the bond angle larger, which is promoted by H atoms bonding with top Si atoms.