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