The directional property of soils, particularly clays, are directly related to the particle orientations. The platy clay particles do possess the tendency to reorient themselves in the most stable condition against applied stress. One may conceive of developing a soil model directly relating particle orientation and applied stress. In this study an angle θ, which is the preferred (therefore stable) orientation of clay particles, is introduced. Using this concept and slip-line theory, the velocity equations of clay in a plane-strain condition are developed. In this approach any change in clay particle orientation is essentially dependent on principal stresses magnitudes as well as their directions. Therefore, the effect of rotation of principal stresses can be included and quantified.