Geomembrane liners in landfills fulfill the important role of avoiding groundwater and soil pollution by preventing leakage of leachate. In order to maintain the effectiveness of the liners for a long time, they should be tolerant against physical and chemical factors prevailing in the environment under which they are used. However, due to the lack of durability data obtained from the field samples, little is known about the actual durability of liners in landfill sites. In this study, 66 geomembrane samples in total, which comprise 27-year-old PVC, 24-year-old TPO, 18-year-old EPDM, 12-year-old HDPE, etc., were taken from six different landfill sites, one test site of landfill liners, and one test site of pond liners. Tensile tests were carried out along both the Machine (MD) and Transverse (TD) direction, and retention ratio of tensile strength (=amount of change/initial × 100), retention ratio of extension at break (=amount of change/initial × 100), and retention ratio of thickness were obtained. Using the above results, the changes in the physical properties of the liners were evaluated.