The long-term durability of HDPE geomembrane for waste landfill leachate barrier system was assessed by the analysis of stress relaxation behavior and chemical stability. The yield strain of HDPE geomembrane was about 8％ in the tensile testing according to ASTM D638 method. In this study, the 6％ strain was applied to examine stress relaxation behavior through this tensile test. By evaluation of relative stress at the accelerated condition, the temperature dependence on the stress relaxation was examined. The relative stresses were decreased with increaing temperature. Master curve was obtained by using time-temperature superposition principle and the long-term performance of geomembrane at 10$^{8}$ minute could be predicted. The relative stress due to chemical deterioration varied in the range of -5~10％ at different exposure time and temperature.