Carbon fiber (CF) is recognized as an optimal reinforcement for the composites application areas which light weight and high mechanical properties are needed. In all manufacturing processes, the surface of the CF is subject to various defects for the following reasons: unevenness of the polymer chain orientation due to friction between the nozzle wall and the PAN resin in the spinning process, unbalance of force applied during stretching and thus surface cracking, uneven reaction with oxygen during stabilization, surface cracks due to fiber volume shrinkage during carbonization process. These various surface defects of CF remain as obstacles to the development of high performance fibers. Therefore, in this study, surface defects and surface layer etching of CFs were tried using super-heated steam, and the mechanical strength changes of CF according to surface etching were investigated.