Homopolymers and copolymers were synthesized by polycondensation and copolycondensation, with varying feed ratios of bis(3-hydroxypropyl terephthalate) (BHPT) and bis(4-hydroxybutyl terephthalate) (BHBT) at 270°C. In addition, in the mol ratio of 1:1, copoly(trimethylene terephthalate/butylene terephthalate) [P(TT/BT)], with reaction times of 5, 10, 20, 30, and 60 min, was synthesized to identify the chain-growth process of the copolymers. From differential scanning calorimetry (DSC) data, it was found that a random copolymer might be formed during copolycondensation. The molecular structure of copolymers, formed through the interchange reaction of BHPT and BHBT, was investigated using carbon nuclear magnetic resonance spectroscopy ( 13 C-NMR). We calculated the sequence-length distributions of trimethylene and butylene sequences and randomness in the copolymers using 13 C-NMR data. From the values of the number-average sequence length calculated, it was determined that a random copolymer was produced: This result coincides with previous DSC data. The lateral spacing of the unit cell of the copolymer increased slowly when the mol percent of one monomer was increased to that of the other monomer, indicating broadening of the unit cell by lateral distortion.