A new compound was synthesized by chemical combination of (3-mercapto)propyl-heptaisobutyl polyhedral oligomeric silsesquioxane (POSS-SH) and 1,3:2,4-bis(3,4-dimethylbenzylidene) sorbitol (DMDBS) via epichlorohydrin while hydroxyl groups were still retained in the product POSS-DMDBS. The prepared POSS-DMDBS was introduced into isotactic polypropylene (iPP) to improve crystallization behaviors of iPP and obtain nanocomposites with suitable mechanical properties. Crystallization and mechanical properties of iPP/POSS-DMDBS were systematically investigated by wide-angle X-ray diffraction, polarization microscopy, atomic force microscopy, differential scanning calorimetry, and tensile tests. The spherulite size of the modified iPP was obviously decreased with the addition of POSS-DMDBS, while the crystallization temperature was increased by 5C to 9C depending on the content of POSS-DMDBS incorporated. POSS-DMDBS exhibited relatively higher nucleating efficiency on iPP which is similar to that of DMDBS, confirmed by the increased crystallization temperature. It was also found that the tensile modulus of iPP after adding POSS-DMDBS increased significantly with respect to pristine iPP, but the elongation values decreased. Introduction of POSS-DMDBS in content less than 1 wt% could bring about effective influence on the crystallization behaviors of iPP, demonstrating its potential applications.