The influence of ZnO nanoparticles on the crystalline structures of nylon-6 under different crystallization conditions (annealing at different temperatures from the amorphous solid, isothermal crystallization from the melt at different temperatures, and crystallization from the solution) has been examined with differential scanning calorimetry (DSC), wide-angle X-ray diffraction, field emission scanning electron microscopy, and Fourier transform infrared. ZnO nanoparticles can induce the gamma-crystalline form in nylon-6 when it is cooled from the melted state and annealed from the amorphous solid. This effect of ZnO nanoparticles increases with decreasing particle size and changes under different crystallization conditions. The effects of ZnO nanoparticles on the crystallization kinetics of nylon-6 have also been studied with DSC. The results show that ZnO nanoparticles have two competing effects on the crystallization of nylon-6: inducing the nucleation but retarding the mobility of polymer chains. Finally, the melting behavior of the composites has been investigated with DSC, and the multiple melting peaks of composites containing ZnO nanoparticles and pure nylon-6 are ascribed to the reorganization of imperfect crystals. (C) 2003 Wiley Periodicals, Inc.