A lot of geotextile reinforced soils are designed and constructed in Japan where earthquakes happen frequently. Therefore, the stability of the geotextile reinforcement soil during earthquakes is very important. In general, the decrease of reinforcement due to creep is considered in ordinary (non-seismic) design. On the other hand, in seismic design, it is reasonable to use the tensile strength retained after a certain period of load history. In this research, a series of tests was conducted to investigate the residual strength and creep performance of a geogrid made frompolyester fiber. This paper describes the validity of the allowable strength of geogrid and proposes the extra coefficient of allowable strength of geogrid for seismic design.