For an ac plasma display panel (PDP), the lifetime should be guaranteed over 30,000 h. The lifetime is correlated with the deterioration characteristics of the weakest element in an ac PDP. In this paper, a relative lifetime test using higher operating voltage and higher sustain frequency than those of normal condition is proposed in order to investigate the lifetime of MgO thin films with accelerated erosion. By this method, the effects of the MgO thin film thickness and the deposition rate are investigated on the relative lifetime of an ac PDP. The relative lifetime increases with MgO thin film thickness, but it is almost saturated with thickness over 5000 Å. Also, the lifetime decreases with the increase of the MgO deposition rate and increases with Xe fraction in the working gas of He+Ne+Xe.