Most fibers are irregular and are often subjected to rapid straining during mechanical processing and end-use applications. In this article, the effect of fiber dimensional irregularities on the dynamic tensile behavior of irregular fibers was examined using the finite-element method (FEM). Fiber dimensional irregularities are simulated with sine waves of different magnitude (10, 30, and 50% level of diameter variation). The tensile behavior of irregular fibers was examined at different strain rates (333, 3333, and 30,000%/s). The breaking load and breaking extension of irregular fibers at different strain rates were then calculated from the finite-element model. The results indicate that strain rate has a significant effect on the dynamic tensile behavior of an irregular fiber, and that the position of the thinnest segment along the fiber significantly affects the simulation results. Under dynamic conditions, an irregular fiber does not necessarily break at the thinnest segment, which is different from the quasi-static results. (C) 2004 Wiley Periodicals, Inc.