Polyethylene/montmorillonite (PE/MMT) nanocomposites with different dispersion states of MMT were prepared by in Situ polymerization. Isothermal crystallization of the intercalated nanocomposite, in which the PE chains were confined in the MMT layers, was studied and was compared with that of the exfoliated nanocomposite. It is observed that the intercalated sample has longer induction period, longer crystallization half time and larger crystallization activation energy than the exfoliated sample, showing that crystallization of PE is retarded due to confinement of the MMT layers. Analysis of crystallization kinetics shows that Avrami exponent (n) increases gradually with crystallization temperature. However, the maximal value of n is 2.0 for the intercalated sample. but it can reach 3.0 for the exfoliated sample. It is inferred that the sterns of the PE crystals confined in the MMT layers are parallel to the MMT layers. The Hoffman-Weeks extrapolation method cannot be applied in the intercalated sample because of the small lateral surface of the PE crystals. Based on the depression of the melting temperature, the specific free energy of the PE/MMT interface was estimated, which is about 1.0 mJ/cm(2), much smaller than the free energy of the lateral surface of PE crystals. This is attributed to the origin of the strong nucleation effect of MMT. (c) 2005 Elsevier Ltd. All rights reserved.