The differential isoconversional method of Friedman is applied to non-isothermal melt crystallization DSC data to obtain effective activation energy ΔE. In comparison to neat PET, ΔE of intercalated 93A MMT clay nanocomposites (PCNs) decreased and highly dependent on the clay content. Hoffman-Lauritzen (H–L) secondary nucleation theory parameters, Kg and U* were evaluated using isoconversional approach of Vyazovkin and Sbirrazzuoli. Crystallization regime transition from regime III is observed at 190°C for neat PET, which shifted to higher temperature range 200°C–208 °C for PCNs. The Kg parameters for both regimes for PET are consistent with our isothermal experiments and reported in literature. However, the Kg values of nanocomposites are highly sensitive to the temperature dependent nucleation activity and ΔE of PET chain motion in melt and are not very much comparable with our isothermal results. Nevertheless, the observed results clearly indicate that the 93AMMT clay layers predominantly act as heterogeneous nucleating sites.