Carbon nanofibers were mixed with polypropylene powder by sonication method and extruded into filaments with a single screw extruder. Tensile tests were performed on the single filament at a strain rate range from 0.02/min to 2/min. Results indicate that tensile modulus and yield strength of materials increased with increasing strain rate. Results also show that infusing polypropylene with nanofibers increases yield strength and modulus, but decreases ductility. These increase in strength and modulus are due to the well dispersion and alignment of CNFs.