Previous work at Kimberly-Clark Corp. (R. S. Nohr and J. G. MacDonald, Kimberly-Clark Corp., unpublished results, 1990) demonstrated that the addition of an ultrafine-particle( submicron size), surface-modified silica predispersed in an alkyl silicone to isotactic polypropylene (iPP) results in dramatic improvements in the tensile properties of fibers and spunbonded fabrics. For both fibers and spunbonded fabrics prepared with a high concentration of the low melt flow index (MFI) resin the incorporation of the additive resulted in markedly improved tensile properties. In this article it is shown that under quiescent conditions the surface-modified silica/silicone copolymer additive acts as an effective nucleating agent for IPP. This results in a reduction in the size of the crystalline entities, hence a more homogeneously distributed crystalline phase and load-bearing "tie" molecules. Indeed, scanning transmittance electron microscopy (STEM) studies show that the crystalline entities are substantially smaller at the surface and mid-radius of fibers prepared with additive containing iPP. Furthermore, the bond points of the resulting spunbonded fabrics have a dramatically increased nucleation density and smaller crystallite dimensions.