Polyamide 66 (PA 66) nanofiber bundles were first electrospun and then introduced into isotactic polypropylene (iPP) melts to prepare nanofiber bundle/iPP composites. To reveal the influences of matrix molecular weight (M (n) ) on the transcrystalline layer, three kinds of iPP with different M (n) were adopted. Polarized optical microscope was employed to investigate the transcrystallinity. In the presence of PA 66 nanofiber bundle, the heterogeneous nucleation distinctly happened in iPP melts. Moreover, the higher the iPP M (n) , the denser the nuclei. Both a decrease in matrix M (n) and an increase in isothermal crystallization temperature led to an increase in the induction time. The maximum temperature at which the transcrystalline layer can be optically observed increased with the increase of M (n) . The growth rate of transcrystallinity decreased with the increasing M (n) and crystallization temperature. Moreover, selective melting of the transcrystalline layers confirmed that it was merely composed of alpha form crystal for all composites.