Electrospun nylon-6 fibers were prepared from its polyelectrolyte solution in formic acid with different concentrtaions. In situ Fourier transform infrared (FTIR), wide-angle X-ray diffraction and small-angle X-ray scattering (SAXS) were performed on the nylon-6 fibers heated to various temperatures until melting. For comparison, stepwise annealing of the solution-cast film having exclusively the alpha-form was also carried out to elucidate the structural evolution. Our results showed that Brill transition in the electrospun fibers occurs at a lower temperature than that in the solution-cast film due to the crystal size difference. Differential scanning calorimetry heating traces on the as-spun fibers exhibited a unique crystalline phase with a melting temperature of similar to 235 A degrees C, higher than the equilibrium melting temperature of nylon-6. The content of high melting temperature (HMT) phase increased with increasing nylon-6 concentration; a maximum of 30 % of the fiber crystallinity was reached for fibers obtained from the 22 wt.% solution regardless of the heating rates used. Based on the SAXS and FTIR results, we speculated that the HMT phase is associated with thick alpha-form crystals developed from the highly oriented nylon-6 chains that are preserved in the skin layer of the as-spun fibers. A plausible mechanism for the formation of the skin/core fiber morphology during electrospinning was proposed.