Plasma display panel(PDP) have gained great attention due to their potential application to large-area display including HDTV. The luminance and luminous efficiency of PDP, however, should be improved to realize this goal. In this study, we examined experimentally the effects of phosphor thickness and discharge gap on the luminance and luminous efficiency of ac PDP. For the rib height of 110 ${mu}m$, whereas the optimum phosphor thickness was about 30 ${mu}m$. The optimum thickness of green phosphor was about 50 ${mu}m$ for the rib height of $120{sim}160;{mu}m$.