The luminescence efficiency of Mn4+-doped CaAl12O19 (Mn:CAO) is significantly improved by composition modification. The leading mechanism that quenches the Mn4+ photoluminescence is the formation of Mn4+ pairs in the lattice of CaAl12O19 (CAO) with interstitial O2- for charge compensation. Mixing Mg2+ ions into the CAO lattice may form Mn4+-Mg2+ pairs and reduce the number of Mn4+ pairs, thus enhancing the Mn4+ luminescence efficiency by more than three times. It is shown that the presence of Mg2+ leads to formation of additional phases that also affect the optical properties of Mn4+.