The luminescence efficiency of the YAG:Tb phosphor has been improved by the application of flux to the firing process. Luminescence efficiency is largely determined by the single‐phase crystallinity of cubic YAG:Tb. This phosphor, when used in a projection CRT, shows good stability in the temperature and current dependence of brightness as well as good resistance to electron burning. The effect of on the formation of single‐phase cubic YAG and the optical performance of the resulting YAG:Tb phosphor in a CRT are discussed.