Aromatic-aliphatic copolyesters of poly(ethylene-co-diethylene terephthalate)-co-poly(L-lactic acid) (PEDLT) with different compositions were synthesized by direct melt copolyesterization of L-lactic acid oligomers and poly(ethylene-co-diethylene terephthalate) (PEDT) oligomers. Chemical structures, thermal properties, and crystalline behaviors were studied. H-1 NMR confirmed that the short lactyl units were incorporated into the PEDT chains. Thermogravimetric analysis showed that PEDLT copolyesters were thermally stable up to about 380 degrees C. Both wide-angle X-ray diffraction and Fourier transform infrared spectroscopy showed that no poly(L-lactic acid) crystal was formed during the crystallization of PEDLT copolyesters. Isothermal crystallization of PEDLT copolyesters was carried out, and it was found that both the crystallinity and crystal growth rate decreased after incorporation of lactyl units compared with those of PEDT. The obtained aromatic-aliphatic copolyesters are expected to find a potential application as biodegradable polymer materials.