The effects of graphene oxide (GO) on the crystallization and crystal structures of nylon 6/nylon 6-grafted graphene oxide (GO(graft)) nanocomposites (NGO) during heating, cooling, and cold drawing experiments were studied with in situ wide-and small-angle X-ray scattering measurements at a synchrotron radiation source, and compared to those of pure nylon 6. For the liquid-nitrogen-quenched samples, the small concentration of GOgraft in the nylon 6 matrix (0.0045 wt %) induced the formation of alpha-phase nylon 6, while the pure nylon 6 exhibited the beta phase. The degree of crystallinity and long period of the NGO increased from 10.0 to 17.5 % and from 7.31 to 10.6 nm, respectively, as the GOgraft content increased from 0 to 0.045 wt%. During the heating process, the a phase of the NGO changed to a new beta phase at approximately 150 degrees C through a Brill transition, and then to a high-temperature alpha' phase at around 195 degrees C. On the other hand, the initial beta phase of the nylon 6 only transformed into the high-temperature alpha' phase at 192 degrees C. When stretched at room temperature, both the beta phase of the virgin nylon 6 and a phase of the NGO were transformed into a metastable phase, and the fraction of this metastable phase increased at the expense of the beta (or alpha) phase. (C) 2015 Elsevier Ltd. All rights reserved.