The effect of anodic oxidation on high-strength polyacrylonitrile-based carbon fibers has been studied in terms of fiber surface energetics and fracture toughness of the composites. According to contact angle measurements based on the wicking rate of a test liquid, anodic oxidation leads to an increase in surface free energy, mainly due to the increase of its specific (or polar) component. For the carbon-fiber-reinforced epoxy resin matrix system, a direct linear relationship is shown between the specific component and the critical stress intensity factor measured by the single edge notched beam fracture toughness test. From a surface-energetic point of view, the anodic treatment may be suitable for carbon fibers incorporated in a polar organic matrix, resulting in an increased specific component of the surface free energy. Good wetting plays an important role in improving the degree of adhesion at interfaces between fibers and matrices of the resulting composites.