We report the investigation on the oxidation behavior of Si_1－xGe_x alloys(x=0.05,0.15,and 0.25). In the PL spectra of germanium quantum dots formed by the oxidation of Si_1－xGe_x substrate,it is clear that there are several peaks at 724 nm,769 nm,810 nm and 861 nm wavelength along the emission band,which are correlated the quantum confinement effect with 3.98 nm,4.17 nm,4.35 nm and 4.62 nm diameter of the germanium clusters,respectively. The simulation result with MC method demonstrates that the germanium clusters in above diameters are more stable under above conditions. A quantum confinement model has been set,and some calculations with the UHFR method and the quantum confinement analysis have been proposed to explain the PL spectra.