olymeric composites based on the combination of polymers (both thermoplastic and thermoset) and mineral fillers, metals, and fibers have found a wide range of applications over the past 40 years. Recently, the tremendous potential for property enhancements when nanoscale particles, such as layered silicates or carbon nanotubes (CNTs), are incorporated into polymers has led to an explosion of research activity in polymer-based nanocomposites. Polypropylene (PP) is one of the most important commodity polymers. It is extensively used in industry to manufacture bottles, films, fibers, etc. Therefore, a great deal of effort has been made to modify its mechanical properties. The goal of our research program is producing multi-walled carbon nanotubes (MWCNT)-polypropylene composite fibers via straightforward melt spinning and studying mechanical properties and morphology of the composite fibers. Composites were produced by dry blending of PP granules with a given ratio of multi-walled carbon nanotubes (MWNTs) using a twin screw extruder. Fibers were melt spun using a melt spinning apparatus. Some of the process variables were investigated e.g. mixing time (rotor speed), concentration of nanotube and fiber’s draw ratio. Physical properties and structure of the composite fibers were investigated using Several techniques such as tensile testing, Thermal Gravimetric Analysis (TGA), Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), Differential Scanning Calorimetry (DSC), birefringence and nanoindentation. The results from measurements of helical content ratio of the fibers indicate an increase in the ordered region of the fibers with the increase in nanotube concentration, rotor speed and fiber’s draw ratio. The molecular orientation of the fibers was increased with increase in nanotube concentration, rotor speed and draw ratio. DSC and XRD results showed an increase in overall crystallinity of the composite fibers compared to pure PP fibers. Tensile tests showed a 7% increase in breaking stress of the composite fibers (1 wt% MWNT). Modulus was increased about 17% in the composite fibers (0.5 wt% MWNT) compared to pure PP fibers, too. Young’s modulus and the breaking stress of drawn composite fibers increased about 15% with increase in rotor speed of the mixing twin extruder. Increasing draw ratio of the composite fibers caused an increase in fiber’s Young modulus and breaking stress, too. Composite fiber’s hardness was increased with increase in nanotube concentration. Experimental mechanical results were compared with some theoretical models such as Krenchel’s rule and Halpin-Tsai for elastic modulus and Pukanszky’s model for yield stress.