Morphological study on the poly(ethylene 2,6-naphthalene dicarboxylate) (PEN) fibers, which were prepared at spinning speeds ranging from 2 to 10 km/min, was per-formed by combining the surface etching techniques with the surface observation methods. From the results by scanning electron microscopy (SEM) of the etched fiber surfaces, it was clarified that well-developed fibrillar structure which runs continuously along the fiber axis exists in the high-speed-spun (HSS) PEN fibers spun at a spinning speed beyond 4 km/min. Fine network structure was observed on the lateral surface of each fibril by SEM. From the results by atomic force microscopy (AFM), it is proposed that several crystalline lamellae are closely stacked in each of the constituents which form the fine network structure. (C) 2005 Elsevier Ltd. All rights reserved.