Long-term performance of geogrid should be dependent on the mechanical behaviors during the service periods. Mechanical properties of geogrids are mainly influenced by not only creep deformation behaviors at the constant confining load and temperature but also stress relaxation behaviors at the constant strain and temperature as viscoelastic deformation. In this study, new concept mechanical model was suggested and applied to analyze the long-term performance of geogrid with instantaneous and asymptotic responses. Experimental results of creep and stress relaxation tests of geogrids were compared each other to verify this suggested mechanical model.