MgO pellets are provided for use as a protective layer for a plasma display panel providing improved physical properties. The plasma display panel includes first and second substrates facing each other. A plurality of first and second electrodes are internally formed on the first and the second substrates. Dielectric layers cover the first and the second electrodes and a MgO protective layer covers one of the dielectric layer. The MgO protective layer has 400 columnar crystals per .mu.m.sup.2.