Variation of wall voltage and priming effect due to an auxiliary electrode are investigated with the aim of attaining a better understanding of the discharge phenomenon in an ac plasma display panel with an auxiliary electrode. A periodic positive pulse, which is applied to the auxiliary electrode during afterglow, reduced wall charges accumulated on the auxiliary and sustain electrodes so that discharge current decreased with an increase of auxiliary-pulse voltage. In addition, the auxiliary pulse influenced on the priming particles so that the optical output was not changed as much as the reduced current and sustain voltage was lowered. Based on these results, modified auxiliary-pulse waveforms, i.e., dual auxiliary pulses, were applied to the auxiliary electrode to utilize more priming particles to improve luminous efficacy. The proposed auxiliary waveforms improved IR efficiency by 10%-15% as compared to that obtained with a single auxiliary pulse.