This paper deals with the effects of activation process of MgO thin film on the dielectric characteristics of surface discharge type AC PDP. The discharge voltage decreased with increase in the MgO activation time and temperature until $400^{circ}C$ under the condition of clean high vacuum. However, for the sample activated above $430^{circ}C$ the discharge voltage increased. These results may be explained by the MgO morphology change and surface cleaning in the process of activation.