Finding a Suitable plasticizer for polylactide (PLA) is necessary to overcome its brittleness and enlarge its range of applications. In this study, commercial PLA was melt-blended with a new plasticizer, an ethylene glycol/propylene glycol random copolymer [poly(ethylene glycol-co-propylene glycol) (PEPG)] with a typical number-average molecular weight of 1.2 kDa and an ethylene glycol content of 78.7 mol %. The thermal properties, crystallization behavior, and mechanical properties of the quenched blends and the properties of the blends after storage for 2 months under the ambient conditions were investigated in detail. The advantage of using PEPG is that it does not crystallize at room temperature and has good compatibility with PLA. The quenched PLA/PEPG blends were homogeneous and amorphous systems. With an increase in the PEPG content (5-20%), the glass-transition temperature, tensile strength, and modulus of the blends decreased, whereas the elongation at break and crystallizability increased dramatically. The cold crystallization of PLA resulted in phase separation of the PLA/PEPG blends by annealing of the blends at the crystallization temperature. After storage under the ambient conditions for 2 months, the PLA/PEPG blends retained good toughness but still lost some flexibility. The reasons for the aging of the blends were the enthalpy relaxation and cold crystallization of PLA and the induced phase separation. Poly(ethylene glycol) with a typical number-average molecular weight of 10 kDa was also blended with PLA for comparison. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114:1105-1117, 2009