A series of polypropylene (PP) blends were toughened by various impact modifiers with different mechanical characteristics. The impact, tensile dilatometry and dynamic mechanical behavior of these PP/high impact polypropylene (HIPP) blends was studied. The results showed that the impact strength of PP improves significantly with the addition of a higher content of HIPP which has the lowest flexural modulus and yield strength. This was due to the formation of shear yielding bands in the slow crack growth zone ahead of the notch tip. However, the PP blends modified with HIPP of higher flexural modulus exhibited much lower impact strength owing to these dispersed particles initiate crazes in the slow crack growth zone.