Crystallization behavior of isotactic polypropylene (iPP) with extremely high isotacticity has been studied over a very wide range of crystallization temperature T-c (145 degreesC less than or equal to T-c less than or equal to 166 degreesC). Optical polarized microscopy observation reveals that morphologies obtained before and after T-c = 157.0 degreesC are different in nature. Differential scanning calorimetric measurement shows that the melting temperature (T-m) increases significantly with increase of both T-c and crystallization time t. A sudden jump in the, slope of T-m versus log t plot is observed at T-c = 157.0 degreesC, which suggests a kind of order-disorder transition taking place at high T-c (greater than or equal to 157.0 degreesC) to allow better chain sliding diffusion. Such a disordering has been confirmed by means of X-ray measurements and will be reported in another paper of this series. It was also convinced that annealing at high temperature subsequent to lower temperature crystallization promotes much more rapid lamellar thickening than crystallization alone does at the same high temperature. The lamellar thickness is determined by transmission electron microscopy and also shows a discontinuous upward shift at T-c = 157.0 degreesC. The lamella with the thickness up to 66 nm is reported for the first time.