The influence of the poly(alpha-pinene), a natural polyterpene, on growth rate of isotactic polypropylene spherulites was investigated using a dynamic solidification method. It was shown that the use of a constant cooling rate allows the obtainment of reliable growth rate data and permits one to enormously enlarge the range of temperature where crystallization data can be measured. The presence of poly(alpha-pinene) slows the radial growth rate of polypropylene crystals for all blend compositions investigated (0-50% poly(alpha-pinene)). Analysis of the experimental data with the Hoffman-Lauritzen model showed that a regime II-III transition is present for plain iPP and for the iPP/P alpha S miscible blends and that the transition temperature decreases with poly(alpha-pinene) content.