Poly(l-lactic acid)-poly(ethylene glycol)-poly(l-lactic acid) triblock copolymer (PLLA-PEG-PLLA) prepared by simple esterification of l-lactic acid (l-LA) and diol-polyethylene glycol (diol-PEG) is proposed as an effective plasticizer for poly(lactide) film (PLA). Differential scanning calorimetry and X-ray diffraction analyses reveal improvement in PLA/PLLA-PEG-PLLA blend miscibility without PEG segment recrystallization in the PLA matrix. A comparative study on the performance of PLLA-PEG-PLLA with two different PLLA chain lengths clarifies that long PLLA-PEG-PLLA with balanced block lengths of PLLA and PEG segments performs a semicrystalline structure of PLA crystal without PEG-crystal formation, resulting in blend miscibility. This microstructure clearly induces the perfect alpha-form of the PLA crystal with increased degree of crystallinity to 50%. Additionally, the sufficiently long PLLA segment in triblock copolymer can get entangled with PLA chains, allowing to enhance melt strength of the PLA/long PLLA-PEG-PLLA blend. In contrast, short PLLA-PEG-PLLA is amorphous, obstructing the formation of the highly ordered PLA packing structure in the blend, and contributing to poor melt strength and mechanical weakness. Furthermore, the PLA/long PLLA-PEG-PLLA blend improves toughness with increased elongation at break to 80% while maintaining the relatively high tensile strength, as compared to that of PLA/PEG or PLA/short PLLA-PEG-PLLA films at the same blending weight ratio.