The effects of the incorporation of different types of fibers on the crystallization kinetics and thermodynamics of isotactic polypropylene (IPP) are investigated. The study is mainly performed by thermal analysis, both in isothermal and constant cooling rate conditions, utilizing differential scanning calorimetry (DSC). The crystallization kinetics of the studied composites is successfully described by the Avrami model. Moreover, the effects of different kind of fibers are studied comparing the values of kinetics parameters such as the Avrami exponent, n, the kinetic constant Of the crystallization rate, K-n and the half-time of crystallization, tau (1/2). The results of the investigation show that the fibers behave as effective nucleant agents for the crystallization of polypropylene. In fact, a dramatic decrease of the half-time of crystallization, tau (1/2), as well as a sensible increase of the overall crystallization rate, Kn are observed In the presence of all the fibers analyzed. These effects are more marked in the presence of aramid fibers. The Avrami model is successfully applied to describe the crystallization kinetics of the composites. The kinetic curves obtained under non-isothermal conditions confirm the results obtained under isothermal conditions and demonstrate the nucleating action of the fibers on the PP crystallization. Furthermore, the spherulite growth and the transcrystallinity on the surface of the fibers are investigated by Optical polarizing microscopy. It is observed that transcrystallinity takes place in all kind of fibers studied, but also in this case, aramid fibers are the most effective in promoting transcrystallinity.