Modification of the rheological and thermal properties of polypropylene (PP) was accomplished through the in-situ generation of PP/poly(trimethylene terephthalate) (PTT) nanocomposites fibrils containing organoclay via a melt spinning process. The development of the fibrillar structures in the fibers during the melt spinning and a subsequent hot drawing process was confirmed by scanning electron microscopy; the lowest mean diameter of the nano-fibrils was 74 nm for the hot drawn nanocomposite fibers consisting of 10 wt.% of PTT and 1 wt.% organoclay in the PP. The results of wide-angle X-ray diffraction and differential scanning calorimetry indicated that the presence of the 10 wt.% of PTT and 1 wt.% organoclay in the nanocomposite fibers led to an approximately 16% increase in the crystallinity of the PP, compared to pure PP fibers. The distribution of the organoclay in the nanocomposites was investigated using transmission electron microscopy and small-angle X-ray scattering.