Carbon nanofibers with diameters in the range of 100nm to 300nm were obtained by stabilizing and carbonizing electrospun polyacrylonitrile (PAN) precursors. The morphologies and structures of the nanofibers and PAN precursors were investigated by scanning electron microscopy, scanning tunneling microscopy, and differential scanning calorimetry. The diameters of the PAN precursors and carbon nanofibers showed a log- normal distribution. The cyclization exothermic peak shifted to a lower temperature for electrospun fibers, which suggested that cyclization could be more easily initiated. Pits 10 nm in length and 5 nm in width formed on the surface of the carbon nanofibers, caused by the rough surface of the electrospun precursors and their shrinkage during heat treatment.