Nanocomposite fibers of polypropylene and montmorillonite-based organoclay were produced by a melt-spinning process, and their structures and mechanical properties were studied. The addition of nanoclay in polypropylene increased the rate of crystallization and altered the microstructures of the fibers. Increases in the crystal size and a reduction in the molecular orientation were observed in the nanoclay-polypropylene composite fibers. The tensile properties of nanoclay composite fibers were also studied, and decreases in the fiber modulus and tenacity and increases in the strain at break were observed. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 410-419, 2011