Optically transparent plasticized poly(lactic acid) (PLA) based bionanocomposite films intended for food packaging were prepared by melt blending. Materials were plasticized with 15 wt% of acetyl(tributyl citrate) (ATBC) to improve the material processability and to obtain flexibile films. Poly(hydroxybutyrate) (PHB) was used to increase PLA crystallinity. The thermal stability of the PLA–PHB blends was improved by the addition of 5 wt% of cellulose nanocrystals (CNC) or modified cellulose nanocrystals (CNCs) synthesized from microcrystalline cellulose. The combination of ATBC and cellulose nanocrystals, mainly the better dispersed CNCs, improved the interaction between PLA and PHB. Thus, an improvement on the oxygen barrier and stretchability was achieved in PLA–PHB–CNCs–ATBC which also displayed somewhat UV light blocking effect. All bionanocomposite films presented appropriate disintegration in compost suggesting their possible applications as biodegradable packaging materials.