Erosion of soils is one of the main mechanisms causing the failure of earth construction. Modelling erosion phenomena is based on the following equation [Hanson, 1990]: dJ/dt = kD(τe – τc) or      kD e c   ; (1) where dJ/dt (or   ) represents the mass (or volume) of soil eroded by water by unit of time and surface, kD the erosion coefficient, τe the effective shear stress at the soil-water interface and τc a threshold shear stress, J the distance between the nozzle of jet and the surface of soil. The questions remain unresolved concerning the use of this equation: are kD and τc reliable parameters for characterizing the “erodibility” of the soil independently from the type of test and from the test parameters. In order to gain a better insight into the role of test parameters and their effect on the erosion parameters, a device “Jet Erosion Test” was used to characterize the erosion of soil. The study was performed on two soils: a natural clayey silty and a mixture of clay and sand.