In this study, we investigated the effect of an organosilane treatment on the surface properties of flax fibres and the resulting mechanical properties of flax fibres reinforced poly(lactic acid) biocomposites. Optimizing the organosilane treatment conditions increases the hydrophobicity of the fibres, and improves significantly the stiffness, the ultimate stress and the impact strength of the biocomposites, while reducing largely the scattering of their properties. Dynamic mechanical analysis reveals a decrease in damping of treated biocomposites because of the formation of a thicker layer of immobilized macromolecular chains resulting from strong interactions at the interface. Crack propagation is observed on notched samples by in situ tensile tests in a scanning electron microscope. The treated biocomposites show a cohesive failure at much higher loads which highlights the enhanced load transfer from the PLA matrix to the flax fibres.