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 ail five oxidation states of Sin+ (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 Is binding energy due to contamination on the SiO2 surface, the core level of the oxidation state Sill 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 similar to 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.