Provided are a protective layer made of magnesium oxide containing at least one rare earth element selected from the group consisting of the rare earth elements, in which the content of the at least one rare earth element is from about 5.0×10−5 to about 6.0×10−4 per 1 part by weight of the magnesium oxide, a composite for forming the protective layer, a method of forming the protective layer, and a plasma display panel including the protective layer. The protective layer can reduce a discharge delay time and the temperature dependency of the discharge delay time, and thus, is suitable for single scan and an increase in Xe content.