Poly(ethylene terephthalate)/(PET) organically modified montmorillonite (OMMT) nanocomposites were synthesized by applying interlayer in situ polymerization. The OMMT was modified using 1-decyl-2-methyl-3-dodecyl imidazolium salt. Application of such an OMMT prevented its degradation process as previously experienced at high temperatures. Micro and submicron structures of nanocomposites were investigated using XRD, TEM, and AFM. Thermal properties and nonisothermal crystallization behavior were analyzed by TGA and DSC, respectively. A correlation between thermal stability and state of dispersion was obtained. Moreover, the effects of different OMMT loadings and cooling rates on nanocomposite samples were also considered. Nonisothermal crystallization behavior was investigated by means of modified Avrami's equation, which showed a crystallite tridimensional growth in nanocomposite samples. (C) 2008 Wiley Periodicals, Inc.