Electrospun nanofibers of 10 semi-crystalline polymers were obtained. The nucleating abilities of these nanofibers toward isotactic polypropylene (iPP) were investigated. Conventional PTFE and PBO fibers were also examined for comparison. The tested fibers were embedded in the iPP matrix to induce the transcrystalline layer and their nucleating abilities were characterized by optical microscopy. However, due to the limitation of optical resolution the increasing number of iPP nuclei on the tiny electrospun fibers was too difficult to trace for determining the surface nucleating rate. To address this problem, the variation of depolarized light intensity under a constant cooling rate was monitored. The induction temperature (T-i) for fiber-surface nucleation was determined to characterize the nucleating ability of fibers; that is, the higher the T-i, the better the nucleating ability of the fibers. Our results indicated that the nucleating ability of fibers can be classified into four categories: strong for the PBT and PTFE fibers; medium for the PIT, PET, iPS, PVA, PA6 and PBO fibers; weak for the sPS fiber; and inactive for the PA46, and PMP fibers. All active fibers induced the alpha-form of iPP crystals except the iPS fibers, which induced the beta crystals through the alpha-to-beta growth transition mechanism. (C) 2016 Elsevier Ltd. All rights reserved.