Reduction factor by creep deformation test of geogrids was interpreted to determine the long-term design strength. For this, limit creep strain of geogrids was analyzed to consider the material strain property. Limit creep strain of geogrids in this study was not over $10\%$ during 10000 hours. Sherby-Dorn plots were applied to compare the real limit strain criteria (GRI Test Method GG-4). Through Sherby-Dorn plots of creep deformation behaviors of geogrids. It was seen that the limit creep strain was $11\%$. This value is higher than the existing criteria of $10\%$. From this limit creep strain 11$\%$, reduction factor from the creep deformation test was 1.45 during 10,000 hours for geogrids. Finally, creep reduction factors of $10\%;and;11\%$ limit creep strains were compared and analyzed to interpretate the difference of reduction factor $0.06~0.14.$