Random copolyester namely, poly(ethylene terephthalate-co-sebacate) (PETS), with relatively lower molecular weight was first synthesized, and then it was used as a macromonomer to initiate ring-opening polymerization of L-lactice. H-1 NMR quantified composition and structure of triblock copolyesters [Poly(L-lactic acid)-b-poly(ethylene terephthalate-co-sebacat)-b-poly(L-lactic acid)] (PLLA-PETS-PLLA). Molecular weights of copolyesters were also estimated from NMR spectra, and confirmed by GPC. Copolyesters exhibited different solubilities according to the actual content of PLLA units in the main chain. Copolymerization effected melting behaviors significantly because of the incorporation of PETS and PLLA blocks. Crystalline morphology showed a special pattern for specimen with certain composition. It was obvious that copolyesters with more content of aromatic units of PET exhibited increased values in both of stress and modulus in tensile test. (c) 2007 Wiley Periodicals, Inc.