This work prepared poly(ethylene terephthalate)/clay (PET/clay), PET/poly(ethylene glycol-co-1,3/1,4-cyclohexanedimethanol terephthalate) (PET/PETG), and PET/PETG/clay nanocomposites via the twin-screw extrusion process. The organoclay in PET and PET/PETG matrices are in homogeneous dispersion, and the spacing distances between the platelets in the PET/clay and PET/PETG/clay nanocomposites are larger than 33.9 angstrom. Differential scanning calorimetry (DSC) and polarizing optical microscope (POM) analyses showed that the inclusion of organoclay significantly increased the crystallinities and melting temperatures of PET/clay and PET/PETG/clay nanocomposites with less time required to undergo isothermal crystallization. These findings imply the occurrence of heterogeneous nucleation. However, amorphous PETG substantially decreased the crystallinity of the crystallizable units of PET. Morphologically, the inclusion of organoclay drove the crystallites of PET from a spherulite with more perfection and less dimension to a larger spherulite. This process was extended from PET/clay to PET/PETG/clay in which the crystallites propagated in two-dimensional and disc-like morphology. However, blending PET with amorphous PETG imparted the larger crystallites with relatively distorted Maltese-cross patterns. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 128: 487-497, 2013