Interfacial adhesion between carbon fiber and epoxy resin plays an important role in determining performance of carbon-epoxy composites. The objective of this research is to determine the effect of fiber surface treatment (oxidization in air) on the mechanical properties (flexural strength and modulus, shear and impact strengths) of three-dimensionally (3D) braided carbon-fiber-reinforced epoxy (C-3D/EP) composites. Carbon fibers were air-treated under various conditions to improve fiber-matrix adhesion. It is found that excessive oxidation will cause formation of micropits. These micropits are preferably formed in crevices of fiber surfaces. The micropits formed on fiber surfaces produce strengthened fiber-matrix bond, but cause great loss of fiber strength and is probably harmful to the overall performance of the corresponding composites. A trade-off between the fiber-matrix bond and fiber strength loss should be considered. The effectiveness of fiber surface treatment on performance improvement of the C-3D/EP composites was compared with that of the unidirectional carbon fiber-epoxy composites. In addition, the effects of fiber volume fraction (V-f) and braiding angle on relative performance improvements were determined. Results reveal obvious effects of V, and braiding angle. A mechanism was proposed to explain the experimental phenomena.