The gamma --> alpha crystalline phase transition in nylon 6/clay nanocomposite prior to melting was investigated by X-ray diffraction. The phase transition in the nanocomposite took place at 160 degreesC, 40 degreesC higher than that of nylon 6 at 120 degreesC. The transition extent in the nanocomposite was lower than that in nylon 6. This could be caused by the strongly confined spaces between layers, and the favorable environment for the formation of the gamma phase in the existence of clay. Besides, the less grown crystallites of the a phase transformed from the gamma phase in the nanocomposite began to melt at much lower temperature than its normal melting temperature.