Geogrid is one of the new materials for reinforcing soils. However, the reinforcement mechanism of geogrid is not very clear and lack of testing study up to now. It means that the theoretical work is far backward to the engineering practice. The interface characteristics between geosynthetic and soil are studied with direct shear friction test and pull-out friction test. Two kinds of geogrids (plastic geogrid and warp-knitting geogrid) are used as the reinforcing materials to strengthen the granular soils (sand, gravel and residual soil). Meanwhile, for comparison purpose, another kind of material--woven geotextile, which is early used in engineering, is also selected to reinforce those granular soils. The test results show that the geogrid is much better than woven geotextile in aspects of shear strength, elongation ratio, and interface friction coefficient, especially for the composition of geogrid and granular soil owing to the synthetic action of friction, interlock and built-in effects between them. Finally, comparing with direct shear test, the pull-out test seems to be a better way to reflect the comprehensive effect above mentioned.