We have studied discharge characteristics of MgO nano-powdered functional layer in accordance with their particle sizes in AC-PDP. The MgO nano-powder's size was controlled and confirmed that the different particle sizes were distributed according to the sintering times throughout the particle size analyzer (PSA) measurement. We made the different 6 inch test panels, in which the different particle size's functional layers were applied, for measurements of the firing voltage, secondary electron emission coefficient and cathodoluminescence (CL) spectrum. It is shown that the functional layer makes the firing voltage and sustain voltage low, and the particles with large size have the lowest firing voltage and sustain voltage, which are related to the high secondary electron emission coefficient.