A supercritical carbon dioxide (scCO(2)) assisted annealing technique is adopted to toughen polypropylene (PP) and its random copolymer with 3.8 wt% co-monomer content (PPR). By using this technique, PP with 4-time improvement in toughness or PPR with superior toughness are obtained. Meanwhile, the stiffness of the materials could be maintained. To understand the toughening mechanism, scanning electron microscope, wide angle X-ray diffraction, calorimetry, dynamic mechanical analysis and small angle X-ray scattering characterizations are carried out. It is demonstrated that the toughening mechanism is strongly related to the microstructure evolutions, including decreased chain density in the mobile amorphous phase, improved crystalline phase and improved rigid amorphous phase after annealing. These microstructure changes could result in higher cavitation ability and diminish inner stress in the amorphous phase, which consequently favors improvement in toughness.