Morphologies of extended-chain crystals with different characteristics were observed with scanning electron microscopy (SEM) in the high-pressure crystallized polyethylene terephthalate/polycarbonate (PET/PC) blends. The crystals memorize their nucleation and growth process, which reveal an involvement of different mechanisms simultaneously. The presence of sliding diffusion during crystal thickening is indicated by a wedge shape of some crystals, while bent crystals suggest the occurrence of transesterification in the formation of the large extended-chain crystals. The observation of two morphological features on one group of crystals shows that two mechanisms may work simultaneously. The connection between folded-chain and extended-chain crystals is demonstrated by the S-shaped extended-chain crystals as well as their direct morphological connection observed with SEM. Though transesterification plays the essential role in the formation of the large crystals, which acts in different aspects during the process, the thermodynamic driving force is the enthalpy gain associated with large crystals. This is a high-pressure self-assembly with a coupling between crystallization and transesterification, which may be instructive to grow such large crystals in similar polymer systems. (c) 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3148-3156, 2006.