Geomembranes are an important structure in landfill sealing systems, because they are hydraulically impervious and deterioration resistant. However, the durability evaluation of a geomembrane is mainly based on accelerated weatherability tests. Therefore, the Japan Chapter of the International Geosynthetics Society investigated four types of geomembranes (PVC, HDPE, TPO, EPDM) at six locations in Japan with the objective of evaluating geomembrane durability. First, several geomembrane test samples were collected from each landfill. Then, the samples were evaluated by laboratory tensile tests. Based on the test results, the relationship between total solar radiation and geomembrane elongation is discussed. The results revealed that the geomembrane elongation ratio is linearly related to in-situ total solar radiation. Finally, a durability evaluation method of geomembrane is suggested based on the results of this research.