In the current study, poly lactic acid (PLA) composites based on cellulose and chitin nanocrystals were prepared by compounding extrusion using liquid feeding. Triacetate citrate (TEC) was used as additive to facilitate the dispersion and distribution of the nanocrystals as well as liquid vehicle to incorporate them into the extruder. Each nanocrystals material was first dispersed in specific amount of ethanol and then mixed with a fixed amount of TEC in order to prepare nanocomposites with 5 wt% of nanocrystals and 20 wt% of TEC. The prepared nanocomposites were compression molded to films and then characterized. The morphology and the mechanical properties were studied. From the point of view of processing, the liquid feeding of nanocrystals was possible; however some agglomerations were observed but even that the mechanical properties of the plasticized PLA were improved.