We have successfully aligned/dispersed the rod and spherical-shaped Si3N4 nanoparticles in the polypropylene (PP) fibers through melt extrusion process to fabricate polymer nanocomposite (PNC) single fibers. The alignment/dispersion of Si3N4 nanoparticles in PP/Si3N4 PNC fibers has been carried out in a systematic manner to produce uniform single fibers. The PNC fibers were first uniformly stretched and stabilized using a two-set Godet machine. The as-extruded single PNC fibers were tested for their thermal and tensile properties. The test results of PNC fibers were compared to neat PP polymer single fibers fabricated using the same procedure as PNC fibers. These results show that the PNC fibers are much (307%) higher in tensile strength and modulus (>1000%) when compared with the neat PP polymer single fibers. The field emission scanning electron microscope results clearly show the alignment of rod-Si3N4 nanoparticles in polymer matrix. The differential scanning calorimetry results show similar to 12% increase in crystallinity for rod-Si3N4 PNCs when compared with the neat PP single fibers. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 1512-1520, 2011