Isotactic polypropylene (iPP) composites filled with electrospun isotactic polystyrene (iPS) fibers were prepared through solution blending by sonication in a nonsolvent water bath. After optimal sonication treatments, the entangled and surface-fused iPS fibers in the nonwoven fabrics were disentangled and separated to facilitate better dispersion in the as-prepared composites. Composites with different fiber concentrations (0.01, 0.1, and 1 wt%) subjected to various thermal treatments were characterized using DSC, POM, and simultaneous WAXD/SAXS. The effects of cooling rates and crystallization temperatures on the formation of beta-form crystals and lamellar morphology were extensively studied. Results showed that electrospun iPS fibers are novel beta-nucleating agent towards iPP to enhance crystallization rate. A beta-form content of similar to 50% was achieved at a fiber concentration of 0.1 wt%, and the beta-form content increased with increasing cooling rate. Under isothermal crystallization, the maximum beta-form content was found at 120 degrees C, lying between the T-alpha beta and T-beta alpha the former is associated with alpha-to-beta and the latter is relevant to the beta-to-alpha crystal transformation due to the difference in the growth rates of both alpha and beta phases. This finding is consistent with POM observations, revealing that a transcrystalline layer of dense alpha-form nuclei is first induced at the iPS fiber surface for initial crystal growth, followed by alpha-to-beta bifurcation growth to develop highly birefringent beta-form crystals. The average lamellar thickness derived from SAXS results is dependent on the supercooling degree, as well as on the relative content of alpha/beta crystals developed in the composites.