A variety of nanocomposites were prepared using lignin-cellulose nanofibers (L-CNF) and poly(lactic acid) (PLA) via a solvent casting process. Acid hydrolysis and high-pressure homogenization processes were used to produce L-CNF from unbleached kraft pulps. Tensile tests were conducted on thin films, and the nanocomposites containing 3 wt. % L-CNF showed a 32.4% increase in tensile strength compared to that of neat PLA. Dynamic mechanical analysis showed that the tensile storage modulus increased in the viscoelastic temperature region with increasing L-CNF content in the nanocomposites. Thermogravimetric analysis (TGA) showed that all the materials investigated were thermally stable from 25 to 310 ºC. Differential scanning calorimetry (DSC) showed a decrease in the cold crystallization temperature. A positive effect on the crystallization of PLA polymers in the nanocomposites with added L-CNF was observed using DSC and X-ray diffraction (XRD) analysis. The degradation profiles and swelling ratios of the nanocomposites improved.