The effect of both a thermoplastic elastomer (EPDM) and short aramid fibres on polypropylene (PP) crystallization kinetics and tensile behaviour has been investigated. The results have shown that PP/EPDM blends are non-miscible in the melt, and, at low EPDM percentages in the blend (<25%), the particles of EPDM act as nucleating agents and co-crystallize with PP. However, at higher percentages, the EPDM obstructs the mobility of PP chains and hence decreases the PP crystallization rate. Aramid fibres are effective nucleating agents for PP crystallization, giving rise to the phenomenon of PP transcrystallinity on their surface. The tensile behaviour of the blends and composites are in intimate relation with the morphological characteristics of these materials.