The thermal behaviors of poly (ethylene terephthalate) (PET)/SiO2 nanocomposites prepared by cryomilling were studied, by comparing with the cryomilled PET and the raw PET. Cryomilling induced amorphization of crystalline PET matrix and the decrease of PET molecular weight. Cryomilled PET/SiO2 nanocomposites have two abnormal phenomena in the DSC 1st heating thermogram due to the stored energy induced by cryomilling. During DSC cooling process, the dispersed nanometer SiO2 particles play a role of nucleating agent in PET matrix, and the hot crystallization ability of PET/SiO2 nanocomposites improves a lot. Besides, the heat stability of cryomilled PET/SiO2 nanocomposites improves more much during reheating. (c) 2006 Wiley Periodicals, Inc.