A new composition of phosphor material AlON:Er codoped with Mg were prepared by solid-state reaction method. The effect of Mg dopants on upconversion luminescence properties of the AlON:Er phosphor was investigated. At certain concentrations, Mg dopants were found to greatly enhance the red emission intensity of AlON:Er phosphors. From our results, the enhancement of the emission intensity by Mg dopants was discussed in terms of the Er distribution in crystal lattice and crystal symmetry by introducing oxygen vacancy. © 2011 The American Ceramic Society.