This research focuses on the reinforcing efficiency of nanomateterials and the role of the reinforcement's dispersion and orientation on the nanocomposite's flexural and tensile moduli. Polypropylene-based composites reinforced with (i) exfoliated graphite nanoplatelets, xGnp(TM), (ii) vapor grown carbon fibers, (iii) PAN-based carbon fibers, (iv) highly structured carbon black and (v) montmorillonite clay were fabricated by extrusion and injection molding. It was found that graphite platelets are the best reinforcement in terms of flexural modulus whereas PAN-based carbon fibers cause the largest improvement in the tensile modulus. The difference in the reinforcing efficiency during the flexural and tensile testing is attributed to (i) the degree of fiber alignment along the flow direction during injection molding, which is higher in the thinner tensile specimens than in the flex specimens; and (ii) the different deformation modes of the two tests. The importance of good dispersion of the reinforcements within the polymer matrix and of perfect contact between the two phases is emphasized comparing the experimental modulus data to theoretical predictions made using the Halpin-Tsai and the Tandon-Weng models.