The optimum voltage and delay time were measured to measure Vt closed curve in AC type plasma display panel (AC PDP) regardless of the state of charge in a cell. When the voltages are applied to an AC PDP, the discharge occurs and the wall charges are accumulated on the electrodes of the inner cell. For the next discharge, an appropriate voltage should be applied considering the wall charges. Therefore, it is necessary to measure the discharge start voltage according to the state of the wall charge accumulated in the cell or the structure of the panel and the figure showing the potential difference among the three electrodes in terms of two horizontal and vertical axes is a Vt closed curve. In order to measure the Vt closed curve, the previous wall charges are erased by the high voltage and the remaining priming particles should be removed by the idle time. In this paper, the Vt closed curve according to the change of the erase voltage and the idle time was measured, and the optimal conditions were investigated.