Orange and red phosphorescence from Eu3+-activated MO (M = Ca, Sr, Ba) was clearly observed after exciting with ultraviolet (UV) light. The phosphorescence properties for CaO:Eu3+ were investigated with a fluorescence photometer. The phosphorescence was due to D-5(0) to F-7(1) and D-5(0) to F-7(2) transitions of Eu3+ ions and it lasted more than 2 h. It is suggested that trapped-electron and hole centers in the CaO host were induced by the UV light and they were responsible for the generation of the long-lasting phosphorescence.