We investigated the effects of changing the width of the auxiliary electrode in an ac plasma display panel including centrally located auxiliary electrodes between the sustain electrodes, based on analyses of the discharge mode. A panel with auxiliary electrodes was driven by pulse waveforms that included auxiliary pulses applied to the auxiliary electrode immediately after every sustain pulse. The discharge modes are changed consecutively from mode 1 to mode 3-2 when the sustain or auxiliary pulse voltages are increased, which can be characterized as follows: 1) only the sustain pulse discharges are generated in mode 1; 2) one of a pair of the auxiliary pulse discharges is additionally generated in mode 2; and 3) the other auxiliary pulse discharge is also generated in mode 3-1 and mode 3-2. Distribution of the discharge mode changes in accordance with the width of the auxiliary electrode (the focus is on modes 1 and 2 because these modes are effective modes); as the auxiliary electrode is narrowed, the range of mode 1 becomes wide and the range of mode 2 becomes narrow. The wide range of mode 1 is attributed to the fact that the discharge of the auxiliary pulse is suppressed due to a longer gap between the sustain and auxiliary electrodes. The narrow range of mode 2 is caused by the discharge of the auxiliary pulse less affecting the discharge of the sustain pulse. Since the auxiliary electrode provides less of a decrease in the number of wall charges on the sustain electrodes, the voltage margin is improved for the case of the narrow auxiliary electrode. The luminous efficacy is mostly improved in mode 1 for the case of the narrow auxiliary electrode due to the auxiliary pulse with a higher voltage generating a larger number of space charges, as well as in mode 2 for the case of the wide auxiliary electrode due to a short electrode gap rendering a great number of wall charges decreasing.