The nonisothermal crystallization kinetics of poly(ethylene terephthalate) (PET) copolymers modified with poly(lactic acid) (PLA) were investigated with differential scanning calorimetry, and a crystal morphology of the samples was observed with scanning electron microscopy. Waste PET (P100) obtained from postconsumer water bottles was modified with a low-molecular-weight PLA. The PET/PLA weight ratio was 90/10 (P90) or 50/50 (P50) in the modified samples. The nonisothermal melt-crystallization kinetics of the modified samples were compared with those of P100. The segmented block copolymer structure (PET-b-PLA-b-PET) of the modified samples formed by a transesterification reaction between the PLA and PET units in solution and the length of the aliphatic and aromatic blocks were found to have a great effect on the nucleation mechanism and overall crystallization rate. On the basis of the results of the crystallization kinetics determined by several models (Ozawa, Avrami, Jeziorny, and Liu-Mo) and morphological observations, the crystallization rate of the samples decreased in the order of P50 > P90 > P100, depending on the amount of PLA in the copolymer structure. However, the apparent crystallization activation energies of the samples decreased in the order of P90 > P100 > P50. It was concluded that the nucleation rate and mechanism were affected significantly by the incorporation of PLA into the copolymer structure and that these also had an effect on the overall crystallization energy barrier. (C) 2007 Wiley Periodicals, Inc. J Appl Polym Sci 106: 4180-4191, 2007