Carbon nanotubes have excellent mechanical properties such as low density, high stiffness, and exceptional strength making them ideal candidates for reinforcement material in a wide range of high-performance composites. Fibers with increased tensile strengths are produced by employing plasma treatment under various conditions. Tensile strength is improved by at least 35%, relative to an untreated fiber. Methods of making such high strength carbon nanotube fibers via plasma processing are disclosed.