The effect of isothermal crystallization process on the crystal structure of nylon-6 and nylon-6/montmorillonite nanocomposite between 0 and 180 °C is examined using X-ray diffraction method. For the examined process, nylon-6 exhibits a gradual transition from γ- to α-phase with increasing crystallization temperature. The crystals of α-phase expand and a high temperature crystallization peak appears at 2θ=28.5° in X-ray diffraction pattern for nylon-6 crystallized at 180 °C. The addition of montmorillonite improves crystalline-phase transition, and increases the perfection of crystallization.