This article describes a low-temperature thermal stabilization method, for the efficient preparation of polyacrylontrile (PAN)-based carbon fibers with improved mechanical properties. In this method, bundles of regular PAN precursor fibers were firstly heated and stored in air for 30 days at 120 degrees C to cyclise the nitrile groups and oxidize the PAN backbone. A further stabilization above 200 degrees C in air made the pretreated fibers fully stabilized. Structural changes of the as-made PAN fibers were observed by Fourier Transform infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, differential scanning calorimetry, thermogravimetry and dynamic mechanical analysis. Microvoid evolutions of the fibers during stabilization and carbonization process were studied by synchrotron small-angle X-ray scattering. Our results showed that the initiation of the cyclization and oxidation reaction at 120 degrees C not only restricts the disorientation of PAN molecules but also reduces the pyrolysis of molecular chains at higher temperatures in the carbonization process. Hence, preferred orientation of crystallites and char yield increased. Moreover, microvoid defects were significantly reduced, leading to a significant improvement of the mechanical properties (a 16% increment in the tensile strength). (C) 2015 Elsevier Ltd. All rights reserved.