Complicated physical and chemical reactions can occur during the thermal stabilization of polyacrylonitrile (PAN) fibers, and they can be macroscopically reflected by the evolution of tension in the fibers. In this work, PAN fibers were oxidized under different parameters in a continuous production line. The tension in the fibers was examined in detail and found to be influenced greatly by the stretching ratio, temperature, and time, as well as the porosity of the PAN precursors. As the thermal stabilization proceeded, tension with different characteristics could result from various reaction mechanisms. At the initial stage, a higher temperature was helpful for lowering the tension, but the tension increased with an increasing stretching ratio. In a later stage, the tension was dominantly dependent on the cyclization reaction and increased with increasing temperature or time. Under the same stabilization conditions, the tension in low-porosity fibers was higher than that in high-porosity fibers. The microstructures, characterized by high-resolution transmission electron microscopy, provided some direct evidence for the partially stabilized fibers that the stabilization in the crystalline phase was slower than that in the amorphous phase. (c) 2006 Wiley Periodicals, Inc.