A comparative performance study of cellulose and starch nanoparticles on plasticized starch reinforcement has been presented. Starch nanoparticles were obtained by ultrasound and acid hydrolysis, and cellulose nanoparticles were extracted by acid hydrolysis from microcrystalline cellulose and sisal fibers. The nanoparticles were characterized according to the zeta potential, the particle‐size distribution, transmission electron microscopy, X‐ray diffraction, and thermogravimetric analysis. The influence of the addition of these nanoparticles to starch films on the morphology, water vapor permeability (WVP), and mechanical properties of the nanocomposites films were investigated. The cellulose nanoparticles exhibited higher electrical stability than those originating from starch. Acid hydrolysis produced starch nanoparticles with higher crystallinity than ultrasound. All the nanoparticles significantly reduced the WVP. The cellulose nanoparticles significantly increased the tensile strength of the starch films; however, they reduced the flexibility of the nanocomposites. The results of this work support the application of starch and cellulose nanostructures for the development of reinforced biodegradable materials. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47001.