Poly(acrylonitrile) (PAN) solutions in N,N-dimethylformamide were electrospun into webs consisting of 350 nm ultra-fine fibers. The webs were oxidatively stabilized and followed by heat treatment in the range of 700-1000degreesC. Characterization of the microstructure of PAN-based carbon nanofibers was performed by x-ray diffraction, field-emission scanning electron microscopy, electrical conductivity and Raman spectroscopy. The L-c(002) and L-a(10) values were calculated to be 1.85-2.15 and 2.23-3.36 nm, respectively. The L-c(002) and L-a(10) values increased by about 86% and 66%, respectively, when the heat treatment temperature (HTT) was increased from 700 to 1000degreesC. The electrical conductivity of carbonized PAN nanofiber webs increased with increasing carbonization temperature, being 6.8 x 10(-3) and 1.96 S cm(-1) at 700 and 1000degreesC, respectively. The D and G bands from Raman scattering were fitted into a Gaussian - Lorentzian hybridized function, and the crystallite sizes in the nanofibers were evaluated from the R-values determined from the ratios of the intensity of the G band to that of the D band. The domain size of the graphitic layers was in the range 1.6-3.2 nm with higher values at higher HTT. Copyright (C) 2004 John Wiley Sons, Ltd.