It has been proposed that during the stabilization of polyacrylonitrile fibers, the reaction is initiated in the amorphous region and then propagates to the ordered region. However, the evidence is unsatisfactory, and some is quite controversial. It has been confirmed in this study through an examination of the shrinkage behavior and properties of stabilized polyacrylonitrile fibers that the cyclization reaction is initiated at 160 degrees C in the amorphous region and that it propagates to the ordered region at about 200 degrees C. Most of the nitrites can be transferred to a ladder polymer structure when the stabilization temperature is beyond 240 degrees C, and the key temperature during stabilization is 240 degrees C. For a higher degree of cyclization, the stabilization temperature should not be below 240 degrees C. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114:3668-3672, 2009