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