An in-situ experimental was carried out to analyze how the gap between a front and rear panel influenced on the discharging properties of an AC Plasma Display Panel (PDP). Initial firing voltages showed almost constant values except at the position of near the barrier rib as the gap increased. The fluctuations of sustain voltages became smaller dependent on the gap, and seemed to have a linear relation to the filtered IR at 828nm. Observation on the Infra-red indicated the excited or the metastable species could be greatly affected by the rear panel regardless of the gap. Based on these results, we discussed a new method to evaluate dimensional fluctuation of an AC PDP.