Polyurethane/organically modified montmorillonite (PU/O-MMT) nanocomposites were electrospun and the effect of O-MMT on the morphology and physical properties of the PU/O-MMT nanofiber mats were investigated for the first time. The average diameters of the PU/O-MMT nanofibers were ranged from 150 to 410 nn. The conductivities of the PU/O-MMT solutions were linearly increased with increasing the content of O-MMT, which caused a decrease in the average diameters of the PU/O-MMT nanofibers. The as-electrospun PU and PU/O-MMT nanofibers were not microphase separated. The exfoliated MMT layers were well distributed within the PU/O-MMT nanofibers and oriented along the fiber axis. When the PU/O-MMT nanofibers were annealed, the exfoliated MMT layers hindered the microphase separation of the PU. The electrospinning of PU/O-MMT nanocomposites resulted in PU nanofiber mats with improved Young's modulus and tensile strength. (c) 2005 Wiley Periodicals, Inc.