The effect of pressure on the processing and, in particular, on the crystallization kinetics of a polymeric material, is often ignored because of the difficulties involved in the experimental measurements. As a consequence, crystallization behaviors were rarely described by using a model which takes into account the effect of pressure on the crystallization. In this study, experimental data on the dependence of the specific volume on temperature and pressure of isotactic polypropylene using a high-pressure dilatometer up to 150 MPa are reported and discussed. In the complex polymorphism of isotactic polypropylene, the main way to obtain γ structure for a high molecular weight homopolymer is to perform crystallization under high pressure. Isothermal crystallization experiments at high pressures confirmed a relevant fraction of γ-form, as revealed by wide-angle X-ray scattering. From the subsequent heating step after the isothermal crystallization, the melting temperatures at the same pressures have been determined to assess the pressure effect on the equilibrium melting temperature by the Hoffman-Weeks analysis. The crystallization times of isothermal experiments have been analyzed according to the nucleation and growth models. In particular, the pressure effects on both the equilibrium melting temperature and the diffusion process are introduced in the modeling.