The objective is to summarize the material ductility and the toughening mechanisms of the thermoplastic polypropylene blended with two different styrene-ethylene-butadiene-styrene tri-block copolymer (SEBS) at the intermediate and high strain rates. PP and two types of SEBS were blended so that the total rubber amounts were 10, 20 and 30 wt % against PP by the two-step blending procedure. Tensile tests are conducted at the nominal strain rates from 3 × 10-1 to 102 s-1. Phase morphology is investigated to estimate the bi-modal rubber particle size distribution. In addition, the fracture surfaces were observed by scanning electron microscopy (SEM) in order to understand the difference of the toughening mechanism for PP toughened with the bimodal rubber particle size distribution in PP and SEBS blends at various total rubber contents. The large material ductility is obtained in the fracture mechanism of craze bands. The craze bands are obtained in the blend whose total SEBS content is larger than 20 wt%. In addition, the weight ratio of small SEBS particles against total SEBS particles is larger than 20 % and the inter-particle distance of large SEBS particles ranging between 100 and 300 nm are additional condition for crazes bands. The synergistic effect of these rubber particles gives rise to a strong increase in the ductility of these bimodal rubber-particle distributed polypropylene systems.