Moderate amount of cellulose nanofibers (CNFs) is added into poly (lactic acid) (PLA) to improve its brittleness for adapting to the packing of the products. PLA and CNFs were mixed together to prepare composite packaging material. The mechanical properties, light transmittance and fourier transfer infrared spectroscopy (FTIR) were tested, and the surface morphology of the composite packing materials was observed by SEM. The mechanical properties of the composite packaging materials increased after the CNFs was added into PLA. When the mass fraction of the CNFs was 2%, the tensile strength and the impact strength reached maximum. As the percentage of CNFs increased, light transmittance of CNFs/PLA composite film decreased, and the haze increased. But it had a little effect on the visibility as packaging material. In conclusion, CNFs is a nanoscale linear material with a certain length to diameter ratio. It has strong reinforcing and toughening effect.