The effect of unmodified (nat) and azide modified (mod) short organic polyethylene terephtalate (PET) and Nylon 66 (PA) fibers on the thermal behavior and dynamic mechanical properties of an isotactic polypropylene (iPP) has been investigated. The study has been carried out by means of differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The results have shown that both fibers behave as nucleant agents for the crystallization of polypropylene. A noticeable increase of the PP crystallization temperature in the presence of both fibers has been observed. This effect is more sensible with the incorporation of azide groups on the chemical structure of the fibers. On the other hand, the percentage of PP crystallinity (X-c) decreases in the presence of PET fibers and hardly varies with the incorporation of PA fibers. The fibers give rise to a sensible increase of the polypropylene stiffness (storage modulus), whereas a slight decrease of the damping factor (tan delta) has been observed. Furthermore, the PP glass transition temperature (T-g) decreases in the presence of the PET fibers and remains nearly unchanged with the incorporation of the PA fibers. The chemical modification of both fibers gives rise to an additional decrease of T-g as compared with the T-g of the plain PP. The activation energies for the relaxation process increase with the incorporation of both fibers, and this increment is more sensible in the case of the azide treated fibers.