Geomembrane sheets come to be commonly used for liners in waste landfill and are subjected to many kind of long term loads, and their function must be kept the sufficient safety during their serviceable term. In this study, creep characteristics of geomembrane made from High Density Poly Ethylene (HDPE) were experimentally investigated with considering the condition that the embedded geomembrane liners are subjected to differential settlement in base ground of waste landfill. The creep characteristics equation for soil material was applied to geomembrane materials to estimate their long term strength Furthermore, the decreased factor in tensile strength of geomembrane liner due to creep deformation is proposed for actual design.