Achieving high molecular orientation, by increasing spinning velocity and/or postdrawing, is critical to improve the mechanical properties of isotactic polypropylene (iPP) fibers. In this study, we have demonstrated that upon annealing around melting point the molecular orientation of both amorphous and crystalline phases in melt-spun iPP monofilaments is significantly enhanced, in parallel with the improved stability of crystals, because of partial melting and oriented recrystallization. As a result, high level stress and remarkable strain hardening are generated during stretching of annealed monofilaments. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 995-999, 2012