We have investigated the correlation between the secondary electron emission coefficient (gamma) of MgO protective layer and discharge luminous efficiency in alternating current plasma display panel (AC-PDP) by varying the O(2) flowing rates of 0, 10, 20, and 30 sccm. It is found that the secondary electron emission coefficient and the brightness are maximum, and the discharge voltage is minimum at the O(2) flowing rate of 20 sccm. As a result, the discharge luminous efficiency is the highest at the O(2) flowing rate of 20 sccm in comparison with others.