In this study, cold and melt crystallization behaviors of amorphous poly(ethylene terephthalate) (PET)/clay nanocomposites were investigated. Two nanocomposite samples with the same amount of inorganic content were prepared by melt processing using natural montmorillonite (Na-MMT) and organo-modified montmorillonite (org-MMT). Depending on the clay structure, clay dispersion into PET and crystallization behavior of the samples were studied using X-ray diffraction and differential scanning calorimetry methods, respectively. Effects of clay structure and organic groups between clay layers in org-MMT on the melt crystallization kinetics of the samples were analyzed with various kinetic models, namely, the Ozawa, Avrami modified by Jeziorny, and Liu-Mo. Crystallization activation energies of the samples were also determined by the Kissinger and Augis–Bennett models. Exfoliated structures were obtained in the nanocomposite samples prepared with both the Na-MMT and org-MMT. From the kinetics study, it was found that the melt-crystallization rate of the sample prepared with the Na-MMT was higher than that prepared with the org-MMT at a given cooling rate. It can be concluded that organic ammonium groups in the org-MMT decelerate the crystallization rate of PET chains possibly by affecting the chain diffusion and folding. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers