The dielectric layers in AC plasma display panel(PDP) are essential to the discharge cell structure, because they protect metal electrodes from sputtering by positive ion bombarding in discharge plasma and form a sheath of wall charges which are essential to memory function of AC PDP. This layer should have high dielectric breakdown voltage, and also be transparent because the luminance of PDP is strongly correlated this layer. In this paper, we discussed the dielectric breakdown voltage and transparency of the dielectric layer under various conditions. As a result, on the $15mum$ thickness, the minimum dielectric breakdown voltage was 435V and the transmission coefficient was about 80% after $570^{circ}C$ firing process. It can be proposed that the resonable dielectric thickness in AC PDP is $15mum$ because it has about 75V margin on the maximum applied voltage.