The skin-core structure of poly(acrylonitrile-itaconic acid) [P(AN-IA)] precursor fibers in wet-spinning has been analyzed by the means of electron probe microanalyser (EPMA), scanning electron microscope (SEM), and transmission electron microscope. The numerical solution of Fick's second law equations for diffusion in the nascent fiber was obtained by using the Method of Lines. It has been found that [P(AN-IA)] precursor fiber composed of four parts had remarkable skin-core structure. The sheet-like skin, which was compact and homogeneous, had high crystallization and highly oriented structure. However, the core with low crystallization and some voids was loose, somewhat disorderly and unsystematic. Moreover, the precursor fiber had a pillar-like layered structure along the fiber axis. The average thickness of each layer increased gradually from the skin to the endothecium. Meanwhile, a structural model of PAN precursor fiber has been built. (9 2008 Wiley Periodicals, Inc. J Appl Polym Sci 108: 947-952,2008.