A poly(L-lactide)-b-poly(ethylene glycol)-b-poly(L-lactide)(PLLA-PEG-PLLA) block copolymer has great potential for use as a flexible bioplastic. Highly flexible bioplastics are required for flexible packaging applications. In this work, a PEG was incorporated into block copolymer as a plasticizer by solvent casting. PLLA-PEG-PLLA/ PEG blends with different blend ratios were prepared, and the plasticizing effect and miscibility of PEG in block copolymer were intensively investigated compared to PLLA/PEG blends. The results indicated that the PEG was an effective plasticizer for the block copolymer. The blending of PEG decreased glass-transition temperature and accelerated the crystallization of both the PLLA and PLLA-PEG-PLLA matrices. The PEG was completely miscible when blended with block copolymer and it improved thermal stability of the block copolymer matrix but not of the PLLA matrix. Film extensibility of PLLA-PEG-PLLA/PEG blends steadily increased as the PEG ratio increased. These non-toxic and highly flexible PLLA-PEG-PLLA/PEG bioplastics are promising candidates for several applications such as biomedical devices, tissue scaffolds and packaging materials.