Abstract 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)-1×1 surfaces and clean Si(111)-7×7 surfaces. We observed shifts of the vibrational energy of Si–O–Si symmetric bending ( ν O1) and asymmetric stretching modes with increasing the number of adsorbed O atoms. Furthermore e, a different behavior of the energy shifts of ν O1 was clearly observed between the H-terminated 1×1 and the clean 7×7 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 1×1 and the clean Si 7×7 surfaces appears conspicuously in the bond angle of Si–O–Si. This phenomenon indicates that the Si–O–Si structure is relaxed by making the bond angle larger, which is promoted by H atoms bonding with top Si atoms.