By introducing a cohesive fracture model into the Mott momentum diffusion analysis [1, 2], Grady and Kipp deduced a formula for predicting the average size of the fragments during a ductile fragmentation process [1, 3].To quantitatively evaluate the accuracy of the Grady-Kipp formula, in this paper, we numerically simulated the fragmentation processes of an elastic-plastic bar under uniform high strain-rate tension.The average fragment sizes were calculated for a wide range of the prescribed strain-rates and material parameters.It was concluded that the Grady-Kipp model provides reasonably close predictions of the ductile fragment size, though slightly deviations exist in cases when the Grady-Kipps fundamental assumptions do not satisfy.The effect of the periodically distributed defects on the fragmentation and fragment sizes was also investigated using this methodology.