Long term durability of HDPE geomembranes were evaluated by the oxidative induction time(OIT) using differential scanning calorimeter(DSC). For the purpose of assessment of long term performance of HDPE geomembranes for landfill liner system, 3 types of compounded HDPE resins and their geomembrane products were prepared. Density, melt index, carbon black contents, and standard and high pressure OIT were tested for these specimens before/after oven ageing ＆ UV treatment. Density increased while melt index and carbon black contents did not change after oven ageing and UV treatment. OIT values of HDPE resins and their geomembranes with added anti-oxidants and carbon black were compared with those without additives. Anti-oxidant and carbon black contents showed significant effects on OIT of HDPE resins and their geomembranes.