The characteristics of the MgO layer are known to be an important parameter that affects the discharge characteristics in an ac-PDP. In this paper, to improve the MgO characteristics of 50-in. full-HD ac-PDP with He (35%) - Xe (11%) contents, RF-plasma treatments on the MgO layer are examined under various gases for plasma treatment. The resultant changes in the MgO characteristics, including the morphology, roughness, secondary electron coefficient, and firing voltage, were examined in comparison with both cases with and without plasma treatment on the MgO layer. It is concluded that the Ar and Ar > O(2) plasma treatments can enhance the MgO characteristics, such as the roughness, secondary electron coefficient, and firing voltage.