It has been recognized that the incorporation of nanoscale montmorillonite (MMT) layers into polymer matrix enhances significantly the heat resistance of the resultant nanocomposites, especially for nylon-6 (N6)/ clay nanocomposites (NCNs). In the present work, the heat distortion temperature (HDT) of NCNs, including the intercalated N6/Na-montmorillonite (Na-MMT) and the exfoliated N6/organo-montmorillonite (OMMT) ones, have been investigated for both non-annealed and annealed testing specimens in comparison with the neat N6. As expected, the incorporation of MMT obviously improved HDT of NCNs, with the highest HDT value obtained in the N6/OMMT system due to its exfoliated nano-structure. After an annealing treatment at 80 degrees C for 6 hr, the HDT revealed noticeable increase for all the samples, particularly for the intercalated N6/Na-MMT nanocomposite that showed the highest increment of 34 degrees C. Differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and Fourier transform infrared (FTIR) techniques were employed to clarify the origin of the variation in HDT after annealing, and the results suggest that the increases in the crystallinity, the glass transition temperature, and the order degree of hydrogen bonding may account for the noticeable increases in the HDT of the nanocomposites after annealing.