A series of ecofriendly glycerol plasticized starch/polyvinyl alcohol (PVA) bionanocomposite films were prepared by the solution casting method using cellulose nanofibril (CNF) particles as reinforcement fillers. CNF particles were prepared by acid hydrolysis of cotton slivers. The bionanocomposite films had 3, 6, 12, and 18 wt% of CNF loading. The nanostructure of the CNF was confirmed by transmission electron microscopy. The films were characterized by FTIR, water diffusivity property at 93% RH, and thermogravimetric analysis. Diffusivity of the films was found to be significantly dependent on the CNF content of the films and on its dispersion in the blend matrix. Diffusion coefficients of the 6 and 12 wt% CNF-reinforced films were lower than that of the blank film. Incorporation of CNF enhanced the thermal stability of the films by shifting the onset of degradation to a higher temperature and also by increasing the char residue.