Glassy carbon electrodes (GCEs) were activated by means of constant potential oxidation to tailor their surface morphology, functional groups and electrochemical activity. After being oxidized at 1.8 V for 5 min in 0.1 M Na₂HPO₄, thornlike nanostructures were observed on the GCEs’ surface as confirmed by atomic force microscopy measurements. Microscopic infrared spectroscopy revealed that hydroxyl groups were introduced on the surface of the GCEs. Electrochemical tests indicated that the activated GCEs exhibited strong signal enhancement for chemical oxygen demand (COD), an important indicator of water pollution level. Compared with a pristine GCE, an activated GCE greatly increased the oxidation current of COD. The influence of activation conditions such as medium, potential and time on the enhancement effect of the GCEs, was examined. As a result, a sensitive, simple and rapid electrochemical method was developed for COD detection, and the detection limit was 0.33 mg L⁻¹. Finally, an activated GCE was used to detect COD values for numerous water samples, and the results were compared to those obtained using the standard dichromate method.