Microvoid toughening material is an active research area where the application in polymer matrix still needs to be popularized. In this work, microvoids were introduced into an isotactic polypropylene (iPP) matrix by introducing octylphenol polyoxyethylene ether (OP-10) and the iPP/OP-10 samples were annealed at 140 degrees C for 3 h. The results showed that the annealed iPP/OP-10 samples exhibited largely enhanced impact strength compared with the annealed pure iPP and the unannealed iPP/OP-10 samples. A synergistic toughening efficiency was proposed to describe the roles of annealing and OP-10 in toughening iPP. Further results showed that the relaxation behavior as well as the crystallization and melting behaviors of the iPP matrix were not influenced by OP-10. However, apparent changes of the microvoid morphology were observed. The annealed iPP/OP-10 samples exhibited largely increased microvoid numbers or decreased inter-microvoid distance. The toughening mechanisms based on the morphology of the microvoid were analyzed.