Natural fibers are frequently used as alternatives of glass fibers as polymer matrix reinforcement to form composite materials. However, their natural hydrophilicity prevents them from being easily compatible with hydrophobic polymers, which represent the majority of matrices. To solve this, direct fluorination of sized natural fibers (flax fibers sized with DGEBA) was performed. EDX, FTIR and 19F NMR analysis evidenced the chemical grafting of fluorine on DGEBA structure and the ablation of the oxiranes rings on this molecule. Chemical modifications of DGEBA have induced a hydrophobic character of this layer, by reducing at 0 the polar component of surface tension. Thereby, treated fibers are supposed to be perfectly chemically compatible with the hydrophobic polymer matrices (e.g. polypropylene). Additionally, the fluorination time also allows the dispersive component of surface tension and the rugosity of fibers to be tailored in order to perfectly adjust these characteristics and fit with the polymer. Moreover, this chemical modification was achieved without altering the mechanical properties of fibers for short fluorination times.