The possibility of phase transformation toughening is demonstrated by the example of the beta-modification of isotactic polypropylene (beta-iPP), which undergoes beta alpha-transformation (i.e., from hexagonal to monoclinic) during mechanical loading. The resulting alpha-IPP exhibits a higher crystalline density than the initial beta-modification. That, along with the exothermic character of the beta alpha-recrystallization, is responsible for the improvement in toughness. that occurs. The occurrence of this beta alpha-transformation is evidenced by differential scanning calorimetry (DSC). Toughness of the alpha- and beta-iPP is studied and compared with the "essential work of fracture" concept by using static-loaded deeply double-edge-notched tensile (DDEN-T) specimens. The main effect of the beta alpha-transformation is a large increase in the specific plastic work consumed in the necked zone. Light microscopic (LM) and infrared thermographic (IT) pictures reveal that the plastic zone becomes larger and its shape more circular when beta alpha-transformation takes place. It is suggested that the principle of mechanical stress-induced phase transformation from a less toward a more dense crystalline state may be a universal tool for toughness upgrading in semicrystalline polymers.