The crystallization behaviors of poly(ethylene terephthalate) (PET) with monosilane isobutyl-polyhedral oligomeric silsesquioxanes (POSS) were investigated in detail. It is observed that POSS acts as an effective nucleating agent under appropriate concentration in the PET system. Polarized optical microscopy (POM) results showed that POSS improved the nucleation density of PET. Besides, the transmission electron microscopy (TEM) observation demonstrated POSS dispersed mostly at the nanometer scale throughout the PET matrix. The crystal structures of POSS and PET/POSS were analyzed using wide-angle X-ray diffraction (WAXD). Moreover, non-isothermal crystallization kinetics of PET and PET/POSS were studied using Mo's method. The PET/POSS system demonstrated the lower value of F(T) than pure PET, indicating that the lower cooling rate was required to attain a specified relative crystallinity for the system. The comparison of the crystallization activation energy (Delta E) showed that the POSS nanoparticles improved the crystallization of PET through an acceleration of the nucleation process. Furthermore, the isothermal crystallization behaviors were also investigated using classic Avrami method. Overall, POSS can act as a nucleating agent and greatly improve the crystallization of PET through accelerating the nucleation process. [GRAPHICS] .