AbstractPolyethylene terephthalate (PET)/clay nanocomposites (PCNs) containing 1 wt% Cloisite 30B (C30B) were prepared via melt compounding. Modulated differential scanning calorimetry (MDSC) for isothermally crystallized samples revealed that the third endotherm at the highest temperature may be attributed to the recrystalization and melting of crystals, reorganized during heating. The first and second endotherms may be associated with melting of the secondary and primary crystals, respectively. The overall isothermal crystallization rate in PCNs was faster than in the neat resin. Growth kinetics revealed that the work required for chain folding and the equilibrium melting temperature in PCNs were somewhat higher than for neat PET. During isothermal crystallization, the steric hurdles introduced by clay layers lead to a reduction in the transport of the PET chains into crystallites. The effective non‐isothermal activation energy for the PCNs was higher than for PET, possibly leading to less perfect crystals in the PCNs. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers