The rigid-rigid polymer toughening concept, i.e., utilization of an engineering polymer to toughen another engineering polymer, was employed to improve the fracture toughness of isotactic polypropylene (iPP) and to maintain the rigidity and heat deflection temperature of iPP. Noryl Poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) is chosen as the toughener phase to toughen PP. The fracture toughness of PP can be significantly improved by adding rigid thermoplastics PPO without causing any reduction in modulus. Incorporation of a small amount of styrene-ethylene-propylene (SEP) rubber compatibilizer, which helps reduce the size of PPO particles from 15 μm down to about 0.5 μm, further improves the toughness of iPP/PPO blends. Detailed fracture mechanism investigation reveals that Noryl PPO particles help trigger massive crazing in the PP matrix and serve to stabilize the growing crazes. The addition of SEP rubber compatibilizer transforms the crazing dominant damage into shear banding dominant damage in front of the crack tip.