Alkaline-earth metal oxides with 6-coordinated configurations easily react with water because of their deliquescent nature. In this study, the high-pressure phase (8-coordinated) of a cerium-doped strontium oxide (SrO:Ce) phosphor was synthesized by sintering at 1500 ℃ in a reduction atmosphere, and its thermal stability was investigated. The results indicated that the 8-coordinated ultraviolet SrO:Ce phosphor had high thermal stability.