In this article, a continuous stabilization process is used to make nonburning (nonflammable) fibers from polyacrylonitrile (PAN) fibers. The effect of the shrinkage behavior and the stretching process of PAN fibers during the stabilization process on the physical properties, morphology, and flammability of the resultant nonourning fibers is studied for the first time. The higher shrinkage of PAN fibers during the continuous stabilization process is found to increase the diameter, the core proportion, and flammability and decrease the Al value, density, mechanical properties, and formation of oriented molecular chains in the resultant nonburning fibers. The effect of the shrinkage behavior of PAN fibers on the fracture surface of the nonburning fibers is also discussed. The nonburning fibers show a fracture structure radiating from the fiber center to the boundary. The structures are composed of small and fine radial strip-layer–like fibrils. Nonburning fibers developed using an optimum stretch process, not only had increased preferred orientation and density, but also had improved mechanical properties. Those fibers also have sufficient nonflammability. © 1993 John Wiley & Sons, Inc.