Mechanistic simulations of fiber suspensions to predict final fiber orientation distribution in the molding of fiber-reinforced composites have been developed. The mathematical algorithm models fibers as chains of connected rigid beads. Parameters such as fiber volume fraction, number of beads per fiber, and fiber flexibility can be modified to model diverse processing conditions. This article focuses on the extensional flow of short fiber suspensions at high fiber concentrations. Simulation results were compared with predictions obtained with the Folgar—Tucker model. It was found that for the case of short fiber suspensions, fiber orientation is well predicted by the Folgar—Tucker model with the interaction constant C I in close agreement with a model proposed by Phan-Thien et al. It was also found that fiber—matrix separation increases with the increase of fiber aspect ratio. The mathematical model presented in this study provides a tool to predict defects and to model orientation distribution in the processing of fiber-reinforced composites.