Thermally stabilized polyacrylonitrile (PAN) fibers were produced by multi-stage thermal stabilization between 190 and 270 degrees C. X-ray diffraction (XRD) and Fourier Transform infrared (FT-IR) spectroscopy were used to investigate the structure of PAN fibers and the stabilized fibers. A new diffraction peak appears at 2 theta = 23.2 degrees in XRD pattern of the stabilized fibers besides the peaks in that of PAN fibers. The distinct structural changes were also observed from FT-IR spectra, the intensities in the 2,936-2,918, 2,244-2,236, and 1,451-1,446 cm(-1) regions decrease remarkably, and new peaks occur at 1,594 and 1,700 cm(-1) (shoulder-like peak) which correspond to the stretching vibration of C=N and C=O groups, respectively. The ultramicrotomy to prepare samples of PAN fibers and the stabilized PAN fibers was explored to characterize their microstructures by high-resolution transmission electron microscopy (HRTEM). The HRTEM images further verified the microstructural changes of PAN fibers during thermal stabilization, and the detailed information was given that thermal stabilization is accompanied by a new structure forming inside the crystallites in PAN fibers, and the (110) plane of PAN fibers transforms preferentially.