Green/blue light emission with peak position around 2.3—2.4eV at room temperature was observed from Si nanocrystals embedded in silicon oxide films. The effects of thermal annealing on the structure, silicon core level and photoluminescence of silicon oxide films with nanostructures were studied. PL spectra consisted of peaks of 1.86 and 2.38eV, which were independent of annealing temperature Ta. Silicon and SiO2 phases were separated in the annealing temperature range. Both PL intensity and the amount of Si4+ increased repidly as Ta>750℃. From our observation, the origin of green/blue light emission is suggested to be related to the defects at the interface and in the SiOx network.