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Ultrafine fibers or fiber web is an attractive material for its high aspect ratio or porous structure which is welcomed in various applications. In this study, ultrafine fibers (5–10 μm) of styrene–acrylonitrile (SAN) copolymer/isotactic polypropylene (iPP) blends were produced by melt electrospinning, SAN acted as a polymeric nucleating agent (PNA) in iPP fibers. Wide-angle X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, and polarized optical microscopy were used to investigate the morphologies and the crystal structures of SAN/iPP electrospun fibers. The results showed that SAN/iPP melt formed microfibers with different morphologies and crystallinities through electrostatic stretching. The morphological distribution of SAN in iPP fibers depended on the SAN content, and the distribution influenced its nucleating activity and the final crystal structure of SAN/iPP electrospun fibers. After annealing treatment, the molecular chains of iPP in the confined SAN/iPP microfibers disorientated and rearranged, leading to the formation of a mixture of α- and γ-crystal forms. The relative amount of the γ-crystal form depended on PNA's concentration, annealing temperature and annealing time. Melt electrospun iPP fibers prepared in this study were collected as fiber webs that can be used for protective clothing material, filtration media, reinforcement for composites, and so on. POLYM. ENG. SCI., 53:2674–2682, 2013. © 2013 Society of Plastics Engineers