In this work, PEG-mb-PPA (abbr.: PEGPA) copolymers were synthesized via chain extension/coupling of poly(ethylene glycol) (PEG) and poly(1,2-propylene adipate) (PPA) diols, and blends of commercial poly(L-lactide) (PLLA) and PEGPA were prepared via melt blending. The multiblock copolymers were characterized by H-1 NMR and DSC, and the PLLA/PEGPA blends were characterized by DSC, SEM, tensile, and impact testing. Plasticization-dominant (elongation similar to 250%), toughening-dominant (impact strength 33 kJ/m(2)), and plasticized-toughened (250%, 14 kJ/m(2)) blends were obtained by varying the composition and loading of PEGPA. The PLLA/PEGPA blends exhibited much more balanced mechanical properties in comparison with PEG-plasticized and PPA-toughened PLLAs, and the plasticization-dominant blends had superior performance stability in comparison with the PEG plasticized PLLA. The SEM micrographs indicate that the immiscible PPA segment formed elongated microphases and acted as a toughener, whereas the miscible PEG segment acted as a plasticizer of PLLA and a compatibilizer of PPA. Such a synergistic effect of both segments was responsible for the balanced mechanical properties. The improved performance stability was attributed to the high molecular weight of the multiblock copolymers and the depressed crystallization of the PEG segment.