Polyacrylonitrile fibers were heat-treated in air, and a series of stretching experiments were conducted in different temperature zones. The effects of tension on the microstructure of the heat-treated fibers and the tensile strength of the resultant carbon fibers were investigated. The results show the variations in morphological characteristics affected by stretching were different as stabilization proceeded. A possible mechanism of tension on cyclization and, thus, stabilization was elucidated. During stabilization, tension at low temperatures led to a great increase in the tensile strength of the carbon fibers, whereas tension at high temperatures resulted in only small improvement in the tensile strength of the carbon fibers. (c) 2005 Wiley Periodicals, Inc.