We have determined the crystallite size of Twaron fibers heat treated at various temperatures by analyzing the wide angle X-ray scattering (WAXS) data employing a profile analysis technique developed by us. This study indicates that the differential change of size with temperature is less than 1% of the mean value along both (110) and (200) directions. Hence, these fibers are thermally stable and thermal aging is negligible. We have also observed that the lattice strain variation with temperature is negligible. These studies demonstrate the thermal stability of fibers in terms of the changes in the microcrystalline parameters. (c) 2006 Wiley Periodicals, Inc.