Hydrolysis mechanism of long afterglow phosphor and the method to improve the water resistance are presented using SrAl2O4:Dy,Eu phosphor as an example. The phosphor with high brightness and long afterglow is liable to hydrolyze in water to form Al2O3 and Sr3Al2(OH)(12) because of the crystal structure, and thus the luminescent property decreases. A thin silica film is coated on the surface of the phosphor using a sol-gel process. The silica film acts as a barrier between water and phosphor, and thus suppresses the hydrolysis reaction.