Abstract In this article, a flax fiber yarn was grafted with nanometer sized TiO2, and the effects on the tensile and bonding properties of the single fibers and unidirectional fiber reinforced epoxy plates were studied. The flax fiber yarn was grafted with nanometer sized TiO2 through immersion in nano-TiO2/KH560 suspensions under sonification. The measured grafting content of the nano-TiO2 ranged from 0.89 wt.% to 7.14 wt.%, dependent on the suspension concentration. With the optimized nano-TiO2 grafting content (∼2.34 wt.%), the tensile strength of the flax fibers and the interfacial shear strength to an epoxy resin were enhanced by 23.1% and 40.5%, respectively. The formation of Si–O–Ti and C–O–Si bonds and the presence of the nano-TiO2 particles on the fiber surfaces contributed to the property enhancements. Unidirectional flax fiber reinforced epoxy composite (Vf = 35.4%) plates prepared manually showed significantly enhanced flexural properties with the grafting of nano-TiO2.