Ultrafine polyacrylonitrile (PAN) fibers, as a precursor of carbon nanofibers, with diameters in the range of 220-760 nm were obtained by electrospinning of PAN solution using N,N-dimethyl formamide (DMF) as solvent. Morphology of the nanofibers for varying concentration and applied voltage was investigated by field emission scanning electron microscopy (FESEM). The thermal properties and structural changes during the oxidative stabilization process were primarily investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and Fourier transform infrared (FT-IR) and Raman spectroscopy. The nanofiber diameters increase as the applied voltage is increased and they also increase with an increase in the concentration of the polymer solution. It was also concluded that the electrospun fibers displayed a very sharp exothermic peak at 297.34 degrees C. A transition temperature observed by FT-IR and Raman was approximately 300 degrees C, which was closely consistent with the results of DSC and TGA studies. It was also found that oxidative stabilization in air was accompanied by a change in color of nanofibers webs.