The stabilized polyacrylonitrile (PAN) fibers were obtained after heating the precursor PAN fibers under air atmosphere by different procedures. The surface structures and compositions of as-prepared stabilized PAN fibers have been investigated by SEM, SSNMR, XPS and Raman spectroscopy. The results show that 200 degrees C, 220 degrees C, 250 degrees C, and 280 degrees C are key temperatures for the preparation of stabilized PAN fibers. The effect of heating gradient on the structure of stabilized PAN fibers has been studied. The possible chemical structural formulas for the PAN fibers is provided, which include the stable and unstable structure. The stable structure (alpha-type) could endure the strong chemical reactions and the unstable structure (beta- or gamma-type) could mitigate the drastic oxidation reactions. The inferences of chemical formula of stabilized PAN fibers are benefit to the design of appropriate surface structure for the production for high quality carbon fibers. (C) 2017 Elsevier B.V. All rights reserved.