Isotactic polypropylene (iPP) shows both double melting peaks and single melting peak depending on cooling rates and crystallization temperatures. This manuscript reviews the related studies. These melting behaviors can be explained based on two types of spherulite structures. Type I Spherulites have predominant crosshatched lamellae with same thickness in both tangential and radial directions. They form at low crystallization temperatures and show one melting peak at fast heating rates, while double melting peaks at low heating rates because of recrystallization. Type II Spherulites have thinner crosshatched lamellae in center and thicker radial lamellae in peripheral part. They form at high crystallization temperatures and show double melting peaks that can not be eliminated by fast heating rates. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 1520-1526, 2010