Mn4+-activated phosphors, Mg28Ge10O48-delta F delta:Mn4+ (MGFs), can be obtained through an oxygen postannealing process. Analyses of the crystal structure and elemental composition by powder X-ray diffraction (XRD) and electron probe microanalysis (EPMA), respectively, indicated that under an O-2 atmosphere, oxygen atoms were substituted with fluorine atoms in the original MGF structure to leach the fluorine atoms with germanium atoms as GeF4 by oxygen postannealing. The MGF phosphor annealed in O-2 exhibited similar to 1.3 times higher quantum efficiency (QE) than that annealed in ambient air. The Raman spectroscopy results suggested that an increase in the content of the [Mn4+O6] octahedron led to an increase in the QE values. Additionally, the relaxation of lattice defects in the lattice interior and on the surface observed by XRD and X-ray photoelectron spectroscopy (XPS) measurements could explain the change in thermal quenching behavior between the different atmospheres, and the decrease in lattice defects increased the QE. The investigation of MGF phosphors prepared by different processes provides insight into the relationships among the surface and local structures, chemical composition, and photoluminescence properties. The optimized synthetic procedure increases the Mn4+ content and decreases the Mn2+ and Mn3+ contents in the phosphor, which drastically increases the luminescence efficiency.