Carbon nanofiber/polyimide (CNF/PI) composite films were prepared by dispersing CNFs in polyamic acid, the precursor of polyimide, followed by imidization. Because the as-synthesized CNFs were difficult to disperse, they were treated with ball-milling in water and surfactant solution for 12 or 24 hours. MgO microparticles were also added to enhance the ball-milling effect. As-prepared composite films were approximately 0.05 mm in thickness. The volume fractions of CNFs in composites were 0.1%, 0.2%, 0.5%, 1%, 2%, and 5%, respectively. The effect of treatment of CNFs on the tensile strength and electric resistivity of CNF/PI films were investigated.