The nanocrystalline(nc) Ge embedded in SiO2 matrix was attempted to synthesize from thermal oxidation of PECVD deposited ａ-GexSi1-x∶H films.The microstructures of the samples were detected by Fourier transform infrared spectroscopy,Raman spectroscopy and XRD.The results reveal that Ge nanocrystals form during oxidation and their numbers and sizes are varied with different Ge content in the as deposited ａ-GexSi1-x∶H alloy and also changed upon different-oxidation conditions.These characteristics may be due to the selective oxidation of Si in ａ-GexSi1-x∶H alloy and the reduction of hydrogen existing in the samples in view of chemical thermodynamic equilibrium. Some nc-Si samples display visible photoluminescence(PL) at room temperature.Based on Brus, exiton three-dimensional quantum confinement model,the nc-Ge size was evaluated from the PL peak positions and show a good agreement with the result of XRD.All the above analyses suggest that nc-Ge buried in SiO2 matrix can be obtained by the present method and its visible photoluminescence is related to the effect of quantum size on nc-Ge.The observation of nc-Ge with TEM is required to further confirm these facts.