Isothermal crystallization and subsequent melting behavior of mesoporous molecular sieve (MMS) filled poly(ethylene terephthalate) (PET) composites have been investigated at the designated temperature by using differential scanning calorimeter (DSC). The commonly used Avrami equation was used to fit the primary stage of the isothermal crystallization. The Avrami exponents n were evaluated to be 2 < n < 3 for the neat PET and composites. MMS particles acting as nucleating agent in composite accelerated the crystallization rate with decreasing the half-time of crystallization. The crystallization activation energy calculated from the Arrhenius' formula was reduced as MMS content increased. It is shown that the MMS particles made the molecular chains of PET easier to crystallize during the isothermal crystallization process. Subsequent differential scanning calorimeter scans of the isothermally crystallized samples exhibited different melting endotherms. It is found that much smaller or less perfect crystals formed in composites due to the interaction between molecular chains and the MMS particles. The crystallinity of composites was enhanced by increasing MMS content. (c) 2005 Elsevier Ltd. All rights reserved.