In order to reduce parts weight in automotive applications, composites where carbon or glass fibers are substituted by natural lignocellulosic fibers are more and more considered. These materials are usually compounded using classical twin-screw extruders. However, as vegetal fibers are sensitive to high temperatures and mechanical stresses, the compounding process must be adapted and optimized. In the present talk, we will first show using rheo-optical techniques how lignocellulosic fibers from various origins behave in a shear flow. Then, we will propose a systematic study of the mechanisms of fiber breakage, based on experiments with an internal mixer. We will show that the kinetics of breakage can be described by exponential laws, functions of the strain supported by the material. The use of these models allows us to calculate the evolution of the fibers dimensions along the screws during the compounding process and thus to propose an optimization of process conditions and screw profile in order to limit fibers degradation.