The phase stability and melting behavior of nylon 6 were studied by high-temperature wide-angle X-ray diffraction and differential scanning calorimetry (DSC). The results show that most of the alpha phase obtained by a solution-precipitation process [nylon 6 powder (Sol-Ny6)] was thermodynamically stable and mainly melted at 221 degrees C; the double melting peaks were related to the melt of alpha crystals with different degrees of perfection. The gamma phase formed by liquid nitrogen quenching (sample LN-Ny6) melted within the range 193-225 degrees C. The amorphous phase converted into the gamma phase below 180 degrees C but into the high-temperature alpha phase at 180-200 degrees C. Both were stable over 220 degrees C. alpha- and gamma*-crystalline structures were formed by annealing but were not so stable upon heating. Typical double melting peaks were shown on the DSC curve; melt recrystallization happened within the range 100-200 degrees C. The peak at 210 degrees C was mainly due to the melting of the less perfect crystalline structure of the c phase and a fraction of the alpha phase; the one at 219 degrees C was due to the high-temperature alpha- and gamma-phase crystals. (c) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 1885-1891, 2011