Ca0.64WO4:Eu0.24 also crystalizes in the phase of CaWO4 with about 12 mol% of Schottky defects on Ca2+-sites in the crystal lattice of CaWO4. The question whether such a phase is well stable at high temperatures remains to be studied, so the impacts of over-sintering on the structure of Ca0.64WO4:Eu0.24 ceramics are examined. The probable origins resulting in the phase transition at high temperatures are discussed, and the influences of such a phase transition on the luminescence properties are also studied. Observations reveal that some oxygen ions bonded to Schottky defects may be released when the sintering temperature is over 1100 ℃. This leads to the shortage of oxygen element for the bulk Ca0.64WO4:Eu0.24 ceramics, and a phase transition in CaWO4 may have occurred. A monoclinic phase of the formula Eu2 WO6 is generated. It is found that the distance between crystal planes in CaWO4 becomes larger after the phase transition. This may be one of the primary reasons accounting for the sharp decrease of luminescent intensities of Ca0.64WO4:Eu0.24 ceramics.