In an effort to understand the crystallization process of isotactic polypropylene (iPP), especially in the early stages, we performed an on lattice Monte Carlo (MC) simulation. Some small mesomorphic domains composed of relatively ordered helices are found even in the melt state, even though the ordering of helices is very low. Two characteristic temperatures are observed during a stepwise cooling procedure from the melt to a temperature below the melting temperature. One is at ~ 450 K, and the other is at ~ 390 K. The analyses of these two characteristic temperatures support the nucleation and growth mechanism of polymer crystallization.