Polypropylene (PP) compounds with varying amounts (0.4-8.2 vol%) of tailored allyl-isobutyl polyhedral oligomeric silsesquioxane (POSS) were prepared by melt-blending. The dependence of the crystallization behavior and crystalline structure of PP on the melt-state phase morphology of PP-POSS materials is addressed. PP-POSS systems were predicted to exhibit weak phase separation in both the molten and solid states based on Bagley plot using Hansen solubility parameters. Small-amplitude shear rheometry analysis suggested that the highest loaded (8.2 vol%) PP-POSS system behaves as a two-liquid emulsion, whereas the low-content POSS (0.4-4.0 vol%) systems deviate from this model, resembling a polymer nanocomposite. Based on these findings we hypothesized the formation of a heterogeneous phase morphology in the molten state comprised of nano-size "pseudo-solid" POSS clusters and micrometer-size "pseudo-liquid" POSS droplets dispersed within the PP matrix, depending on the POSS content. Upon cooling, POSS droplets comingle, forming cube-like micrometer-sized crystal domains. The nonisothermal crystallization of PP is enhanced by the presence of POSS clusters. Small-angle X-ray scattering analysis revealed the formation of thinner and more heterogeneous folded chain PP lamellae in the PP-POSS systems.