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It is expected from existing theories that the core level of Si nanocrystals (nc-Si) embedded in a SiO2 matrix should shift toward a higher binding energy as compared to that of bulk crystalline Si due to quantum size effect. Indeed, it is observed in X-ray photoemission experiments that the Si 2p core level shifts to a higher apparent binding energy by 1−2 eV for all five oxidation states of Si n + (n = 0, 1, 2, 3, and 4) in the material system of SiO2 containing nc-Si. However, it is found that the core-level shift is due to a charging effect in the material system. After correction for the charging effect by using C 1s binding energy due to contamination on the SiO2 surface, the core level of the oxidation state Si4+ is the same as that of pure SiO2, whereas the core level of the isolated nc-Si with an average size of about 3 nm shifts by ∼ 0.6 eV to a lower binding energy as compared to that of bulk crystalline Si. It is suspected that the core-level shift of the nc-Si toward a lower binding energy is due to the influence of the differential charging between the SiO2 surface layer and the nc-Si underneath.