In this work, phosphorylated cellulosic fibers were functionalized with an aminosilane ((3-aminopropyl)triethoxysilane, APTES) using a simple and economical method. Several characterization were performed to determine the types of bonds between phosphorylated fibers and grafted APTES. The thermal behavior, hydrophobicity and surface charge variation as a function of pH of the multifunctional cellulose fibers were determined. Results demonstrate that APTES should proceed through Si-O-C, and possibly Si-O-P, covalent bonds with cellulose although the dimerization of silane through Si-O-Si bonds has also been observed. The terminal amino groups are expected to be partially involved in hydrogen bonds with phosphate hydroxyl groups found at phosphorylated cellulose fiber surface, causing a pulling in the configuration of the grafted APTES. The two chemical modifications proposed in this work do not significantly modify the morphology of cellulose fibers. XRD analysis also shows that the crystal structure of the phosphorylated fibers did not change after functionalization with APTES. The silylated phosphorylated fibers show potential flame-retardant properties with improved hydrophobicity. Furthermore, the functionalization of phosphorylated fibers with APTES changes the pH of zero charge point from 3.2 to 9.4 and providing a zwitterionic structure suitable for the simultaneous adsorption of both cationic and anionic species.