This paper deals with a novel concept of induction of well-distributed submicrometer voids in polypropylene (PP) and PP/CaCO3 nanocomposites during processing to alleviate their brittleness without sacrificing Young's modulus and yield strength The role of these voids in initiating/participating in the plastic deformation processes during tensile and double-notch four-point-bend (DN-4-PB) tests is investigated It is shown that the voids act in a similar way as the cavitated rubber particles in rubber-toughened polymer systems, that is, plastic growth of the pre-existent voids) in the PP matrix occurs upon deformation and subsequently triggers large plastic deformation of the surrounding matrix in the form of isolated and domainlike craze structures