This paper proposes a relative lifetime test method of MgO thin film. The suggested test conditions are 5$0^{circ}C$, 400Torr, 20% over-voltage and 300KHz. The relative lifetime of MgO thin film is significantly affected by the MgO preparing conditions and Xe partial pressure. As result, the lifetime of the AC plasma display panel (PDP) is increased with an MgO thickness of 2000$AA$ to 8000$AA$ but is saturated over 5000$AA$ (up to 9000 $AA$). In addition, as Xe partial pressure increases, AC PDP lifetime increases.