Ammonium-modified polyacrylonitrile was prepared by free-radical solution copolymerization of acrylonitrile (AN) with ammonium itaconate (AIA), and then it was spun into precursor fibers in this study. Effects of AIA and itaconic acid on the characteristics of the copolymers and precursor fibers were studied in contrast. It has been found that the ammonium modification could obviously increase the hydrophilicity of the copolymer. And in the spinning processes, modified copolymer can withstand higher total draw-ratio of 12.6 folds, while unmodified copolymer only 8.5 folds. The void volume, fineness, tenacity, and elongation at-break of the ammonium-modified precursors are all improved and enhanced. WAXD test manifested that the modified precursors had higher degree of orientation. DSC test showed that the thermal properties of the modified and unmodified precursor fibers were almost the same. (c) 2006 Wiley Periodicals, Inc.