In spite of considerable effort, the understanding of the molecular structure of polyacrylonitrile-based fibers is still incomplete. This paper shows that molecular modeling based on molecular mechanics and dynamics can be used to study the molecular structure of polyacrylonitrile-based fibers with appropriate strategies. Additional information has thus been obtained which is impossible to obtain by using other instrumental techniques. It is concluded that the chain conformations of polyacrylonitrile-based fibers are similar to those of nematic liquid crystal composites in which the chains are formed by the irregular combination of two or more species of unit groups. Hence the polyacrylonitrile molecules should be viewed as ’aperiodic chain molecules.’