Plant fibers sourced from lignocellulosic biomass are increasingly finding use as fillers in biocomposites. However, the mechanical properties of these materials are highly dependent on the morphology of the plant particles. Mastering these proprieties is therefore a key challenge for further development of biocomposites. Among all lignocellulosic biomass feedstocks, miscanthus is an increasingly popular purpose-grown crop. Miscanthus fibers are produced by grinding, and this unit operation affects particle elongation. Here we investigated the influence of three milling technologies generating three different modes of stress on the particle shape factors of two particle populations ([0–0.2 mm[and [0.2 mm–0.5 mm[) selected for their potential end-uses. Stress intensity, defined as the ratio between the screen size used for the incoming powder and the screen size used for the exiting powder, was adjusted by varying the screen sizes. Particle size and elongation factor distributions were measured and then related to the dominant stress mode in the grinder and its intensity. The results highlighted that, regardless of the stress mode in the grinder, lower stress intensity systematically produces more elongated particles, mainly due to fewer breakages in their maximum length. However, at higher stress intensity, the impact of the stress mode on particle morphology becomes more pronounced. High shear stress, as encountered in the centrifugal mill, resulted in more elongated particles, which is the target shape for biocomposite applications