Nanocomposite fibers of nylon-6 and an organically modified montmorillonite (O-MMT), Cloisite-30B, were prepared by electrospinning. Dispersion and exfoliation of O-MMT in nylon-6 were achieved by melt-extrusion in a twin-screw extruder prior to dissolving in aqueous formic acid for electrospinning. The effects of O-MMT layers on the properties of the nylon-6 solution and electrospun nanocomposite fibers were investigated. Homogeneous, cylindrical nanocomposite fibers with diameters ranging from 70 to 140 nm could be prepared from the 15% composite solution. The O-MMT layers were well exfoliated inside the nanocomposite fibers and were oriented along the fiber direction. Both the degree of nylon-6 crystallinity and the crystallite sizes increased for the nanocomposite fibrous mats, most significantly for those composed of the smallest fibers electrospun from 15% solution. The mechanical properties of the electrospun fibrous mats and single fibers depended not only on the addition of O-MMT layers but also on the sizes of the fibers. Smaller fibers exhibited higher Young's modulus. (c) 2006 Elsevier Ltd. All rights reserved.