Deformation and fracture processes of two types of modified polypropylenes (PPs) were investigated in situ by high voltage electron microscopy (HVEM, 1 MV) and scanning electron microscopy (SEM). One type of PP modified with ethylene-propylene block copolymer rubber (EPR) showed a variation in particle size and interparticle distance with content of ethylene. In the other series of modified PP, Al2O3 filler particles with concentrations of 10 and 60 wt % were used. The micromechanical toughening mechanism was comparable in both types. Deformation structures in both systems are closely connected with cavitation and void formation : the systems modified with EPR show void formation inside the modifier particles; in the systems modified with Al2O3, debonding occurs at the interface between the particles and the matrix. Additionally, the effect of the morphology of the modifier particles on micromechanical deformation processes was studied. The experiments showed that besides particle size and center-to-center distance between particles, the ratio of center-to-center distance to particle diameter plays an important role. Models (three-stage mechanism) for the micromechanical deformation process are proposed.