Thermoplastic-lignocellulosic fiber composites are usually prepared by compounding using a twin screw extruder. During the process, the local thermomechanical conditions lead to a severe decrease of fiber dimensions, with a global diminution of length and diameter, thus reducing the potential of mechanical reinforcement of the polymer matrix [1, 2]. In order to optimize the compounding process for maintaining satisfactory fiber dimensions, it is necessary to understand the mechanisms leading to fiber breakage (defibrization). Therefore, experimental data have been interpreted using flow modelling allowing to calculate the local flow parameters, like temperature, specific energy, stress, strain… In the present study, we have considered lignocellulosic fibers of various botanical origins. We have first characterized the changes in fiber dimensions along the screws, by performing dead-stop experiments, collecting samples and analysing length and diameter distributions. Then, we used the modelling software Ludovic© [3] to determine the values of the local flow parameters, all along the screw profile. From evolutions laws of fiber morphology obtained using internal mixer experiments [4], it is now possible to predict the change in fiber diameters and lengths along the extruder. These calculated results have been satisfactorily compared to experimental ones. After validation, the methodology can be used for optimizing either the screw profile (location, number and type of mixing elements) or the processing conditions (flow rate, screw speed, barrel temperature, fiber introduction…). [1] J. Beaugrand & F. Berzin. J. Appl. Polym. Sci., 128 (2013) 1227-1238. [2] F. Berzin, B. Vergnes, & J. Beaugrand. Comp. Part A, 59 (2014) 30-36. [3] B. Vergnes, G. Della Valle, & L. Delamare. Polym. Eng. Sci., 38, (1998) 1781-1792. [4] E. Di Giuseppe, R. Castellani, T. Budtova, & B. Vergnes, Biopolymers 2015, Nantes