The effect of plasma surface modification and ultrasonication time on the mechanical properties of multi-wall carbon nanofiber polycarbonate composites were investigated. The study showed that plasma polymerization treatment of multi-wall carbon nanofibers enhanced the dispersion of the nanofibers in the polymer, and hence improved the mechanical properties of the nanocomposite. The study also showed that the mechanical properties of the nanocomposite depended on the ultrasonic dispersion time. Long term dispersion damaged the surfaces of both untreated and plasma polymerized carbon nanofibers. The interface behavior and strengthening mechanisms are discussed.