The fracture and failure mode of alpha- and beta-isotactic polypropylene (alpha-iPP and beta-iPP, respectively) were studied in high speed (1 m/s) three-point bending tests on notched bars cut from injection-molded dumbbell specimens and compared. The fracture response of the notched Charpy-type specimens at room temperature (RT) and -40 degrees C, respectively, was described by terms of the linear elastic fracture mechanics (LEFM), namely fracture toughness (K-c) and fracture energy (G(c)).K-c values of both iPP modifications were similar, while G(c) values of the beta-iPP were approximately twofold of the reference alpha-iPP irrespective of the test temperature. It was demonstrated that beta-iPP failed in a ductile and brittle-microductile manner at RT and -40 degrees C, respectively. By contrast, brittle fracture dominated in alpha-iPP at both testing temperatures. Based on the fracture surface appearance, it was supposed that beta-to-alpha (beta alpha) transformation occurred in beta-IPP. The superior fracture energy of beta-iPP to alpha-iPP was attributed to a combined effect of the following terms : morphology, mechanical damping, and phase transformation. Results indicate that their relative contribution is a function of the test temperature.