Plant products arouse great interest in many fields (construction, automotive, textiles, etc …). However, some of their characteristics as their flammability represent major disadvantages for some applications. Research is now being conducted in academia and industry to develop effective methods of functionalizing or modifying the surface of these fibers to improve their performance. In this respect, radiation-grafting, based on the use of ionizing radiation (gamma or e-beam), represents an interesting method compared to chemical or enzymatic methods. This technique makes it possible to generate radicals on the fibers allowing the grafting of molecules of interest, thus offering a large spectrum of possibilities for the modification of the fibers [1][2]. Two ways for grafting under ionizing conditions can be used (Figure 1), pre irradiation and mutual irradiation. In any case, many factors condition the success of grafting reactions. In particular, the structure and the composition of the substrate influence the nature of the active species formed, and therefore, the feasibility of the grafting reaction. Some techniques, such as Electronic Paramagnetic Resonance (EPR), helps us to better understand these parameters, which will be the subject of this presentation.