Polyelectrolyte solutions of nylon-4,6 in 99 vol.% formic acid were electrospun, and then the concentration effect on the solution spinnability was studied. The microstructure of the as-spun nanofibers was characterized by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). Based on the solution rheology, the concentration of the entangled regime and the concentrated regime (a... (D) ) were 1 and 10 wt.%, respectively. To prepare bead-free fibers, the minimum polymer concentration used was 10 wt.%, yielding a fiber diameter of 49 +/- 13 nm. The fiber diameter (d (f)) was dependent on the solution viscosity () or the polymer concentration (a...(w)) through the following simple scaling law relation: d (f) similar to aEuro parts per thousand and d (f) similar to aEuro parts per thousand. DSC heating trace on the as-spun nanofibers exhibited double-melting behavior. However, after cooling, the second heating trace showed a single melting peak. WAXD intensity profiles showed that the as-spun nanofibers possessed lamellae with small lateral dimensions, and the lattice parameter difference between a-axis and b-axis was significantly reduced due to the rapid electrospinning process. Both structural features induce the occurrence of the Brill transition of nylon-4,6 in the nanofibers at a much lower temperature of 80 A degrees C than that in the melt-processed film, as-revealed by the temperature-variable WAXD.