The suction is an essential parameter to describe and understand the behavior of unsaturated soils. The ability of unsaturated soils to retain water is quantified by determining the water retention curves (WRC), which express the hydraulic behavior of porous materials such as soil. These curves are determined by subjecting samples to several drying and wetting cycles. The curve during drying path is located above the wetting curve, developing a hysteresis phenomenon [1], and value of content water at a given suction value depends on the path used to reach this point. The aim of this paper is to present a study on the hydraulic behavior of soil, water retention capacity due to drying and wetting cycles, pointing out the hydro-mechanical behavior of unsaturated soils. In the first part, the effect of physical and mechanical properties of soil [32] (initial void ratio, particle size, cohesion, density...) on the water retention is presented. In the second part, a complete numerical model was developed, based on the empirical model of Van Genuchten [18], to model the two boundary curves, and the experimental scanning data were bestfitted using the same theory of Mualem model [13]. This complete model requires 4 parameters. This model has been validated with experimental data on different type of soils: sand [10], [34], U.S. Silica F-95 sand [30].