A series of alkaline earth sulfides based phosphors Ca_ 0.8 Sr_ 0.2 S∶Eu 2+ ,Tm 3+ were covered with a layer of protective coating with alkaline earth fluorides by heating the mixture of phosphor and NH_4HF_2 at elevated temperatures. The coatings were characterized by means of XRD and SEM. The optical properties of the coated phosphors and the influences of the coating on their properties have been discussed extensively. The stabilities of the coated and uncoated phosphors have been compared. A series of alkaline earth sulfides based phosphors Ca 0.8 Sr 0.2 S: Eu 2+, Tm 3+ were covered with a layer of protective coating with alkaline earth fluorides by heating the mixture of phosphor and NH 4 HF 2 at elevated temperatures. The coatings were characterized by means of XRD and SEM. The optical properties of the coated phosphors and the influences of the coating on their properties have been discussed extensively. The stabilities of the coated and uncoated phosphors have been compared.