In this study, modified TiO2 grafted flax fibers were used to reinforce PLA composites. The fibers were oxidized in order to improve the interfacial adhesion of the fibers to TiO2 film. The TiO2 film was created on the flax fiber by a Sol-Gel dip-coating technique. The interfacial adhesion of the fibers to the matrix was investigated qualitatively by microscopy, and quantitatively by measuring inter-laminar shear strength. Impact and tensile tests characterized the mechanical properties of the modified TiO2 grafted flax fiber reinforced composites. Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and Dynamic mechanical analysis (DMA) were used to reveal the thermal and thermo-mechanical behavior of the composites. The hygroscopic behaviors of the composites were determined by the water uptake assessment. The results showed statistically significant increase in adhesion bonding of the modified fibers to the matrix. The modified TiO2 grafted flax fibers increased the impact resistance of pure PLA by three times. The microscopic observations verified the modifications in the mechanical and physical properties of the composites. Moreover, the amount of water sorption decreased by 18% in the modified TiO2 grafted fiber reinforced composite. Display omitted •The thickness of the TiO2 film was 143±29nm, and 380±25nm for non-oxidized, and oxidized flax fibers respectively•The cellulose oxidation prior to TiO2 grafting increased the interfacial adhesion of the fiber to the film.•The impact resistance of the composites increased by at least three times more than pure PLA.•The amount of water sorption decreased by 18% in the modified fiber reinforced composites.