A material for preparing a protective layer for a PDP, which reduces discharge delay time, improves temperature dependency, and has enhanced ion strength; a method of preparing the same; a protective layer formed of the material; and a PDP including the protective layer. More particularly, a material for a protective layer that includes monocrystalline magnesium oxide doped with a rare earth element at an amount of 2.0×10−5−1.0×10−2 parts by weight per 1 part by weight of magnesium oxide (MgO), a method of preparing the monocrystalline magnesium oxide by crystallizing it at about 2,800° C., a protective layer formed of the same, and PDP including the protective layer.