The material ductility and toughening mechanisms under high strain rate are characterized in the polypropylene (PP) blended with two different styrene-ethylene-butadiene-styrene triblock copolymer (SEBS) by the tensile tests at the nominal strain rates from 0.3 to 1.00 s(-1), fracture surface observations, interparticle distances, and the morphological finite element (FE) analyses. It is found that the bimodal-distributed SEBS particle morphology enhances the impact material ductility by craze bands formation, which is caused by the stress interaction between large rubber particles with the highly elongated small rubber particles inside the fibrils of the craze. It is found that there are three conditions for craze bands formation. The first condition is that the total SEBS content is larger than 15 wt%. Second condition is that the weight ratio of small SEBS particles against total SEBS particles should be larger than 0.06. Third condition is that the interparticle distance of large SEBS particles should be larger than 100 rim. In the numerical aspects, the present constitutive law with the craze nucleation and growth can successfully predict the craze bands in the microstructural FE models, leading to the useful procedure for identifying the ductile brittle transition based on the microstructure. The synergistic effect of these rubber particles gives rise to a strong increase in the ductility of these bimodal rubber particle distributed PP systems. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 3941-3953, 2008