Electrochemical oxidation surface treatment of polyacrylonitrile-based carbon fibers (CFs) in 0.5 M ammonium oxalate aqueous solution for 94 s with the electric current density being 0.6 mA cm(-2) resulted in simultaneous improvements of interfacial bonding strength and tensile strength by similar to 8.6% and similar to 16.6%, respectively. The improvements were due to the following reasons: (1) besides creating active surface functional groups, the electrochemical oxidation treatment with optimal processing conditions also generated a suitable electrolytic etching capability, which was powerful enough to remove mechanically weak carbonaceous components deposited on the surface of CFs, while was gentle enough not to distinguishably remove structurally ordered sheath region of CFs; (2) the treatment also refined the graphitic crystallites in the sheath region and created additional crystalline impingements/boundaries that could resist to crack growth; and (3) the electrochemical oxidation that preferably started from sharp edges including the tips of micro-cracks mitigated the stress development and propagation. XRD, SEM, Raman, FT-IR, XPS, and an electronic single-filament tensile tester were used to study the structural, morphological, chemical, and mechanical properties of the CFs before and after the treatment. A theoretical model, namely "physical and chemical dual effects through layer-by-layer electrolytic etching", was proposed to explain the improvements. (C) 2010 Elsevier B.V. All rights reserved.