Abstract: To study the influence of MgO on the microstructure and properties of AlON ceramics, some AlON ceramics were synthesized based on spark plasma sintering technology from Al₂O₃ and AlN powders, analyzing the relative density, fracture surface, grain size, and twin content of AlON ceramics with varying MgO content. The results show that with increasing MgO content, the relative density of AlON ceramics increases initially and then stabilizes, while the grain size decreases first and then stabilizes. Increasing MgO suitably, the grain boundary migration can be slowed effectively, pore elimination can be facilitated and grain growth can be inhibited. Additionally, MgO can promote the formation of twins within the AlON ceramics. When the MgO content reaches 0.30 wt% in the composite sinter aids, the gained AlON ceramics exhibit a compact structure, finely uniform grains, and a certain quantity of twins. These microstructure attributes can ensure the excellent performance of AlON ceramics, with a hardness of 18.19 GPa, a bending strength of 505 MPa, a fracture toughness of 5.75 MPaˑm^1/2, and a transmittance of 79% at a wavelength of 4 µm.