In this study, the morphological evolution of poly(acrylonitrile) (PAN) nanofibers during electrospinning was examined via immersion precipitation of newly electrospun filaments in ethanol, and subsequently associating the fiber morphologies with the electrospinning distances (2 to 10 cm). We have observed that an uneven fiber stretching happened throughout the electrospinning process. A massive filament-thinning took place at the initial stage of whipping instability, and fiber stretching at the later whipping stage was mainly concentrated on the bead sections, leading to improved uniformity of the resultant fibers. This work has provided a new insight into the fiber formation mechanism in the electrospinning process. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 2553-2561, 2010