A series of butylene terephthalate-epsilon-caprolactone copolyesters (BCL) with different hard-segment content were synthesized and their chain structures were carefully investigated by high resolution H-1-NMR spectroscopy. The analysis from H-1-NMR spectra of samples reveals that BCL chain chemical and sequence structure is specific and complicated because of the unavoidable transesterification between BT and CL segments during synthesis process and so BCL copolyesters are segmented copolyesters with certain random property as confirmed by DSC measurements.