In　this　article,　aligned　electrospun　polyacrylonitrile　nanofiber　bundles　were　prepared　as　the　precursor　fibers　to　prepare　preoxidized　nanofibers　through　washing,　drying　densification,　damp-heat　drafting,　and　preoxidation　process.　Effects　of　preoxidation　temperature　and　holding　time　on　appearance　and　microstructure　of　the　preoxidized　fibers　were　studied.　Fiber　density　is　increased　from　1.159　to　1.193　g　cm(-3)　after　drying　densification.　Crystallinity　is　increased　from　22.66　to　45.90%　after　fourfold　drafting.　(-1),　and　holding　time　of　1　h　show　a　sufficient　reaction　degree　of　cyclization　and　crosslinking.　Moreover,　there　is　no　occurrence　of　adhesion　between　fibers.　(c)　2013　Wiley　Periodicals,　Inc.　J.　Appl.　Polym.　Sci.　130:　1158-1163,　2013