An investigation is reported on the coating of the alkaline earth sulfide based red emitting phosphor Ca0.8Sr0.2S:Eu2+, Tm3+ by a thin layer of SiO2 or TiO2 using the sol-gel process. Their alkoxides were used as precursors and were dissolved in an aqueous ethanol medium. EDX (energy dispersive X-ray), SEM (scanning electron microscopy) and powder XRD (X-ray diffraction) were used to characterize the coating. The properties of coated phosphors were also tested; resistances of coated phosphors to moisture and other atmospheric components have been enhanced to a great degree without more loss of their optical properties. When the ratio of oxide ranges from 2% to 5% in weight, the brightness of phosphor has no remarkable change but the stability has been improved greatly.