In order to improve interfacial adhesion between the thermoplastic matrix and the carbon nanofibers (CNFs), two types of functionalization was performed on the CNFs: such as oxidization by using mixed acid or hydrogen peroxide and amino-functionalization. Moreover, polyamide 11 (PA11) based composites reinforced with the functionalized CNFs were fabricated, and tensile properties of the composite were evaluated. The effect of functionalization on the tensile properties was discussed by comparing the experimental results to the rule of mixture. From the results of the Raman spectroscopy, IR, and XPS, the carboxyl groups were introduced to the surface of CNFs by oxidization process, and the amino groups were introduced to the carboxyl groups of oxidized CNFs by amino functionalized process. In the case of the PA11 filled with amino-functionalized CNFs, maximum tensile stress and elongation at break were improved in comparison with the results of the composites filled with neat CNFs.