The role of 1,3:2,4-di(3,4-dimethylbenzylidene)sorbital (DMDBS) in inducing oriented crystallization during the melt spinning of polypropylene (PP) fibers has been investigated, regarding the tensile properties and crystal orientation (WAXS, SAXS) of the fibers. Better orientation and higher Young's modulus were obtained at a content of 0.4% (w/w) DMDBS, under conditions in which the DMDBS nanofibrils are fully developed in the PP melt. It is suggested the nanofibrils, oriented by the shear and elongational flow prior to PP crystallization, serve as nucleation centers for its subsequent crystallization.