The disorder in γ–alon is caused by random arrangement of nitrogen atoms and aluminum vacancies. To understand the properties and electronic structure of γ–alon by theoretical methods, the most reasonable structure model is needed. We examined the site preference of nitrogen atoms and aluminum vacancies by first-principles density functional theory (DFT) calculations on Al24O24N8 and Al23O27N5. The calculated results for Al24O24N8 with the lowest total energy indicated that nitrogen atoms prefer to be far away from each other, rather than in a completely random arrangement. The further investigation on Al23O27N5 shows that the aluminum vacancies tend to possess octahedral sites and coordinate only with oxygen atoms. Evaluated by lattice variances (Da and Dθ) and simulated XRD pattern, the most reasonable structure model of Al23O27N5 has little deviation from the experimental results. The calculated bulk modulus of 200.9 GPa in Al23O27N5 is slightly lower than the experimental value. The electronic structure reveals that the bonds of Al–N and Al–O have partially covalent and ionic characterization, while the covalent bond strength of Al–N is stronger than that of Al–O. The calculated band gap is 3.99 eV, which is much closer to the experimental 4.56 eV than previous suggestions.