Currently, numerous efforts are focused on the use of materials from renewable resources as reinforcement agents in nanocomposites. Among such materials readily available, cellulose nanfibrillated (CNFs) have attracted great interest due to their renewability, biodegradability, and spectacular mechanical properties. The effect of oxidation time on the morphology and the rmomechanical properties of polyvinyl acetate (PVAc) reinforced with cellulose nanofibrils (CNFs) was investigated. Two CNFs, prepared by 2, 2, 6, 6-tetramethylpiperidine-1 oxyl (TEMPO)-oxidation of cellulose, were used and are presenting different morphologies and surface properties. The NFC-5min is more than 1 µm long and presents low surface energy and NFC-2h is 300 nm long and presents high surface energy (Benhamou, et al. 2014). Nanocomposites with various weight ratios of NFCs were fabricated by casting and vacuum-drying method using MeOH as solvent. These nanocomposite films exhibited high optical transparency and the MEB analyses have shown that the NFCs were dispersed homogeniously in the PVAc polymer. On the other hand the tensile strengths, elastic moduli are improved with increasing CNFs content. It appears that the size of the fibers has a greater influence as fiber composites having the larger size has the best mechanical properties. Dynamic mechanical analysis showed that the storage modulus of the CNF/ PVAc films increased significantly with CNF content above the glass-transition temperature of PVAc by the formation of an interfibrillar network structure of CNFs in the PVAc matrix. However, the thermal stability of both families of nanocomposites is lower compared to neat PVAc, based on percolation theory, only the smaller fibers seem to accelerate slightly the decomposition process. The outstanding and effective polymer reinforcement by CNFs results from their high aspect ratio, high crystallinity, and nanodispersibility in the PVAc matrix.