The effect of the properties of geotextile on the separation function and the relationship between the reinforcement and separation function in road constructions were considered. The cyclic loads were applied on the surface of the pavement model in cylindrical mold. The tests consist of two type of series. In the first series, the effects of the properties of geotextiles on the quantity of the fine particles movement through the geotextiles are discussed. And the test results show that the unit weight of geotextile is applicable to estimate the separation function. In the second series, the interaction between the reinforcement and the separation function is considered. The reinforcement is a prime function when the ratio of the applied stress on the subgrade soil to the shear strength of the subgrade soil ( o /cu) is high, however, the separation can be an important function when the ratio is low.