Active research is currently being done on the development of efficient surface modifications and functionalization of natural fibres aiming to improve biocomposites performances. Electron beam and gamma (γ) radiation treatments have been widely used to treat various lignocellulosic biomass with the aim of improving their accessibility to solvents and reagents for their subsequent chemical modification and processing, or for enhancing cellulose enzymatic hydrolysis for the production of ethanol and 2nd generation biofuels. The relevance of ionizing radiation treatments for the modification of natural fibres and their biocomposites is addressed in this review. They can indeed generate radicals or functional groups available for grafting functionalized molecules of interest on natural fibres. The effect of ionizing radiations on the structure of lignocellulosic substrates, and more particularly natural fibres, is discussed. The impact of composite and fibre irradiation on biocomposites properties is detailed. The last part of the review presents insights on advantages of radiation-grafting as an innovative strategy to functionalize natural fibres and improve functional properties of biocomposites. The industrial feasibility and costs of radiation induced modifications are also discussed. Based on literature, it appears that ionizing radiation methods used in suitable and controlled conditions are relevant over other physical and chemical methods developed for the surface modification and functionalization of bio-reinforcements in composite applications.