Polylactide (PLA) was plasticized with a new biodegradable macromolecular plasticizer-poly(diethylene glycol adipate) (PDEGA). The crystallization behavior, miscibility, rheological behavior, mechanical properties and phase behavior of PLA/PDEGA blends were investigated. The PDEGA lowered the glass transition temperature and the cold crystallization temperature. With an increase of PDEGA content, the break strain and impact strength increased significantly. The high break strain of 480 % and the high impact strength of 30 kJ/m(2) were obtained for 70/30 PLA/PDEGA blend. PDEGA was uniformly dispersed in the PLA matrix. The results indicated that PDEGA had a good plasticizing effect on PLA.