Characterization of poly(butyleneadipate-co-succinate) (PBAS) poly(butyleneterephthalate) (PBT) copolyesters resulting from the intermolecular ester-exchange reaction between molten PBAS and PBT have been analyzed using H-1-NMR spectroscopy, differential scanning calorimetry, wide-angle x-ray diffraction, and total organic carbon lab analyzer. Using the assignment of proton resonance due to homogeneous and heterogeneous dyads, the average block lengths were investigated over the entire range of copolymer composition. A decrease in melting temperature was observed with the increase of a terephthalate unit in the composition. The result of x-ray diffraction curve matches well with that of average block length and thermal property. When a rich component is crystallized, the poor component is excluded completely in a crystal formation. The biodegradability in copolyesters also depended on the terephthalate unit in the composition and average block length of the aromatic unit.