Protective magnesia coating on Y2O2S:Eu phosphor powders is formed by a layer-by-layer (LbL) method in aqueous solutions. The phosphor powders are first coated with a negative-charged, anionic polyelectrolyte of ammonium salt of poly(acrylic acid) (PAA-NH4), on which a second-layer, positive-charged magnesium hydroxide coating is then deposited by precipitation and heterocoagulation. A uniform and multiple Mg(OH)(2)/PAA-NH4 bilayer coating on the phosphor powder is prepared by repeating the above coating processes. Protective magnesia coating on the phosphor powders, which is evidenced by insignificant degradation in optical properties after an extended period of electron bombardment, is formed by calcining.