A series of PET/expandable fluorine mica (ME) nanocomposites with different proportions of compatibilizer, 10-[3,5-bis(methoxycarbonyl)phenoxy]decyltriphenylphosphonium bromide (IP10TP) were prepared by in situ polymerization process. The crystallite size, nucleation and the effect of content of compatibilizer on the dispersibility of ME in PET matrix were investigated by XRD and DSC techniques. The XRD results showed a decrease in the crystallite size of ME giving rise to partially exfoliated structure and the crystallite size of PET reduced considerably as compared to pure PET at low content of IP10TP. The DSC results revealed a dramatic increase in crystallization temperature (T-c) and also showed enhancement in glass transition (T-g) and melting point (T-m) at low content of IP10TP. These various findings have been explained based on the miscibility of IP10TP/ME with PET.