In this study, the shear strength performance of geogrids that are used as reinforcement on highway subbase are examined. For this purpose, a specially designed pullout test device is developed and used. The test device is a first to be used in Turkey which has dimensions of 1 m length,1 m width and 0.80 m height. Three different sizes of geogrid dimensions are used in the tests with apertures 3 cm x 3 cm, 4 cm x 4 cm, 5 cm x 5 cm, respectively. Variations of shear strengths of geogrids due to time are investigated. The horizontal displacements of geogrid samples are determined both by image technique and diplacement sensors. It has been observed in tests for all the geogrid types that while soil parameters, vertical pressure loads and pullout speed are maintained constant and the tensile force is applied to geogrids, pullout force increases to a certain peak point, where it starts to decline. It is concluded that geogrid aperture size has a considerable effect on highway subbase shear strength and displacements while the effect on the pullout force is negligibly small.