In our previous study (Ref.1), two-step oxidation by using mixed acid and hydrogen peroxide was performed on the CNFs, and tensile property of the composites reinforced with CNF was improved by two-step oxidation. This was because that the mild oxidation by using hydrogen peroxide in the second step was effective to introduce functional groups including oxygen from the results of XPS analysis. In this study, in the second step of the two-step oxidation, it was investigated whether the functional groups can be introduced more efficiently by irradiating with ultraviolet rays, moreover, optimization of UV-enhanced oxidation conditions was performed. From the results of the XPS analysis, by UV-enhanced oxidation, more functional groups including oxygen were introduced to the surface of CNFs compared to the conventional two-step oxidation. When the UV irradiation time was varied in the second step of the two-step oxidation, the oxygen content increased until 2 hours and then saturated. From the results of tensile tests, the tensile strength of the composited reinforced with CNF treated by UV-enhanced oxidation reached the maximum at 1 hour of UV irradiation and it decreased at the further treatment time. This is because that even in the mild oxidation using hydrogen peroxide, the structure of CNF was destroyed by UV irradiation and the strength of CNF itself was reduced. Ultimately, by applying UV-enhanced oxidation for 1 hour, the tensile strength was improved compared to the conventional oxidation without UV rays. References (1) K. Enomoto, Y. Takahashi: Oxidization of Carbon Nanofiber and Its Effect on Tensile Properties of PA11 Matrix Composite Reinforced with Carbon Nanofiber, Conference Paper of PPS-32(Lyon), (2016)