In recent years, natural fibers reinforced composites have received much attention because of their lightweight, recyclable, nontoxic, low cost and biodegradable properties. However, compatibility between hydrophilic natural fiber and hydrophobic polymer is low. One way to increase the fiber/matrix compatibility is to treat the fiber surface, using hydroxyl groups present on the upper layer of the high tenacity viscose. However, hydroxyl groups are not always accessible on the fibers surface due to impurities. It is thus mandatory to study the various possibilities to determine a good cleaning treatment, which furthermore does not have a negative influence on the mechanical properties of the fibers. In this article, (3-aminopropyl)triethoxysilane (APTES) is used to functionalize the fiber surface after cleaning them, in order to prepare the fibers for reactive injection of polyamide-6. In order to determine the influence of each cleaning/treatment on the physical and chemical properties of the fibers and to determine optimal treatments to increase fibers/matrix compatibility, various characterization methods (XPS, FTIR-ATR, SEM, EDX) were combined to theoretical results obtained by a design of experience.