The effects of pristine and amino-functionalized multiwalled carbon nanotubes (MWNTs) on the crystallization behaviors of nylon-6 were investigated by differential scanning calorimetry and X-ray diffraction. The results indicate the presence of polymorphism in nylon-6 and its composites, which is dependent on the MWNTs concentration and the cooling rate. More MWNTs and slow cooling from the melt favors the formation of a crystalline form. With the increase in cooling rates, the crystallinity of neat nylon-6 decreases, and that of the composites decreases initially but increases afterward. Moreover, the degree of crystallinity of the composites is higher than neat nylon-6 under high cooling rates, counter to what is observed under low cooling rates. The heterogeneous nucleation induced by MWNTs and the restricted mobility of polymer chains are considered as the main factors. Furthermore, addition of MWNTs increases the crystallization rate of a crystalline form but amino-functionalization of MWNTs weakens this effect. The influence of thermal treatment on the crystalline structure of MWNTs/nylon-6 composites is also discussed. A gamma-alpha phase transition takes place at lower temperature for MWNTs/nylon-6 composites than for nylon-6. The annealing peaks of the composites annealed at 160 degrees C are higher than that of neat nylon-6, and the highest annealing peak is obtained for amino-functionalized MWNTs/nylon-6 composites. This phenomenon is closely related to the different nucleation and recrystallization behaviors produced by various MWNT's in confined space. (c) 2006 Wiley Periodicals, Inc.