Because of their specific properties, vegetal fibers are increasingly used as sustainable polymer reinforcements for eco-composites. Nevertheless, their polar character hinders them from being used more frequently at in- dustrial scale due to their incompatibility with mostly dispersive polymers (the cheapest and most common ones). In this study, direct fluorination treatment was carried out to covalently graft fluorine atom at the outmost surface of flax fibers. Such a grafting has been proved by FT-IR, 19F NMR and XPS spectroscopies, and these characterizations allowed to understand chemical change due to the treatment. This chemical modification induced an augmentation of the dispersive character of flax fibers, by significantly lowering the polar component of surface energy without significant change of the dispersive component. Young’s modulus was also maintained. Thereby, treated fibers become perfectly compatible with hydrophobic polymer, and an improvement in the mechanical performance of the resulting composite is expected according to the literature.