Nylon 6-clay hybrid (NCH) is a molecular composite of nylon 6 and uniformly dispersed silicate layers of montmorillonite. We found that the phase with the high melting temperature (HMT phase) in the NCH annealed under elevated pressure. The melting temperature of the HMT phase was 240-degrees-C. Nylon 6 annealed under elevated pressure did not have the HMT phase. Thus, the presence of the HMT phase was characteristic of the NCH. The relative heat of fusion of the HMT phase (heat of fusion of HMT phase / heat of fusion in the pressure annealed NCH) increased with increase in pressure. High-pressure differential thermal analysis (DTA) measurement revealed that the temperature, at which the relative heat of fusion showed a maximum value, was below about 20-degrees-C of the melting temperature of the original NCH under elevated pressure. It was considered that the nylon 6 crystallite near the melting temperature and the molecular mobility under elevated pressure were necessary to the appearance to the HMT phase.