Nanocomposites composed of cellulose nanofiber (CNF) and polylactic acid (PLA) were prepared using a solvent casting method, with the goal of making green nanocomposites. Prior to the incorporation of CNF into the polymer matrix, surface modification of CNF was accompanied by esterification to improve the dispersion of CNF and its interfacial adhesion with the biopolymer. Microstructure, barrier, mechanical and thermal properties of the nanocomposites were studied. Scanning electron microscopy (SEM) micrographs revealed uniform distribution of nanoparticles in the polymer matrix at low contents (1 and 3 wt%), but also that a higher content (5 wt%) of CNF was easily agglomerated. This caused the mechanical properties of the nanocomposites to be reduced. The results of water vapor permeability (WVP) tests showed that the use of acetylated nanofibers had no significant effect on the permeability of films. Tensile strength (TS) and elastic modulus (EM) of nanocomposites with 1 wt% CNF did not show significant changes, however elongation percentage (E) increased by more than 60%. The TS, EM and E changed significantly for nanocomposites with 3 and 5 wt% CNF. Moreover, nanofiber orientation effectively occurred in the PLA matrix. The reinforcing effect of CNF composition with PLA caused a slight increase in glass transition and melting temperatures. However, the nanocomposite films showed a very similar pattern of thermal behavior to that of neat PLA film.