The effects of sustain electrode gap variation on discharge characteristics were investigated in an ac plasma display panel (PDP) with an auxiliary electrode. In this experiment, the following variations were used for the sustain electrode gap of the PDP with an auxiliary electrode: 120, 140, 170, and 200 μm. Static margin, IR intensity, luminance, and power density were measured and investigated in relation to the sustain electrode gap variation. In all the various sustain electrode gaps, when a proper pulse is applied to an auxiliary electrode, luminous efficacy exceeds the levels achieved in the case of a floated auxiliary electrode. In the case of the shorter sustain electrode gaps of 120 and 140 μm, the improved luminous efficacy is mostly due to an increase in luminance. In the case of the longer sustain electrode gaps of 170 and 200 μm, the improved luminous efficacy is mostly due to a decrease in power density. As the sustain electrode gap increases, the operating sustain voltage becomes higher. On the other hand, the luminous efficacy increases when a proper pulse is applied to the auxiliary electrode.