Nylon6/clay nanocomposites with 5-wt% montmorillonite (NCN5) were prepared by a twin-screw extruder. The effects of thermal histories including annealing temperature (195 degrees C, solid-state annealing; 245 degrees C, melt-state annealing), annealing time and cooling rate on the polymorphic behavior and thermal property of NCN5 have been studied using Modulated Differential Scanning Calorimetry (MDSC) and wide angle X-ray diffraction. It was found that longer annealing time and faster cooling rate favored the formation of the c crystal when NCN5 samples were annealed at 195 degrees C. As the annealing temperature was elevated to 245 degrees C, the a crystal became the absolutely dominating crystalline phase independent on the annealing durations and cooling rate. Moreover, a small endothermic peak was observed around 195 degrees C in NCN5 samples and this new transition peak was also found to be dependent on the thermal history.