This study investigates the issues involving reinforcement of isotactic polypropylene (iPP) spun fibers by molecular adducts originating from the synergistic interactions of polyhedral oligomeric silsesquioxane (POSS) containing silanol functionalities (silanol–POSS) and di(benzylidene)sorbitol (DBS). The molecular adducts of silanol–POSS and DBS were low viscosity liquids at fiber spinning temperature, turned into cylindrical domains during fiber spinning, and remained as nanoparticles in the fibers. The fibers were analyzed by differential scanning calorimetry, wide-angle X-ray diffraction , scanning electron microscopy, and transmission electron microscopy. It was observed that iPP compounds with 2–5 wt % silanol–POSS and 1 wt % DBS could be spun into fibers with close to 40% reduction in diameter compared to unfilled iPP. These fibers offered 60–80% increase in tensile modulus, 50–60% increase in tensile strength, and 100% increase in yield strength compared to unfilled iPP. The silanol–POSS particles were found to be of cylindrical shape with approximately 100 nm in diameter and 200–300 nm in length. The improvements in mechanical properties were correlated with iPP crystallinity and orientation factor.