 The electrochemical oxidation of graphite matrix from the simulative fuel elements for high-temperature gas-cooled reactor was investigated experimentally using NaNO3 solution as an electrolyte. The intercalation and oxidation reactions of graphite were investigated by means of cyclic voltammetry. In addition, the morphological changes of the graphite anodes at predetermined intervals of time during the electro-oxidation process were examined by scanning electron microscopy. The structural transformation of graphite was systematically characterized by different methods. Results showed that the electro-oxidation process induced oxygen-containing groups (i.e., hydroxyl, epoxide, carbonyl/ketone, and carboxyl groups) into the graphite backbone. Electrolytic graphite oxide presented a heterogeneous, indeterminate, and disordered system composed of crystalline and amorphous phases. The structure and microstructure of nuclear graphite, particularly its cracks and defects, primarily determined its destruction pathway during the electrolytic process. The mechanism of graphite lattice destruction could be attributed to the complicated interplay of water electrolysis, anionic intercalation, and gas evolution. The mechanical force caused by gas eruptions among the graphite lattice is the most important and essential factor favoring disintegration.