The crystallization behavior of nylon-6 and nylon-6/montmorillonite nanocomposite films with different heat histories was investigated by wide-angle X-ray diffraction (WAXD). For nylon-6 films isothermally crystallized above 170degreesC or annealed at 200degreesC and then quenched in ice water, a crystalline peak appeared at 2theta = 28.5degrees. This crystalline peak was strong in intensity for the former and weak for the latter. However, for nylon-6 films cooled in air after isothermal crystallization or annealing, no crystalline peak at 2theta = 28.5degrees was observed in the WAXD patterns. For nylon-6/montmorillonite nanocomposite films annealed above 140degreesC, a crystalline double peak was observed between the alpha1 and alpha2 peaks. The possible origins of the peak at 20 = 28.5degrees and the crystalline double peak are discussed.