At a peptide level, electrochemical synthesis offers two methodologies for modification. The first method involves generating an active electron acceptor from a stable precursor via electrode oxidation, which then modifies residues in a peptide through electrophilic substitution reactions. The second method directly generates radical species by single-electron oxidation of a specific functional group in a peptide, which are subsequently captured under suitable reaction conditions to produce a modified peptide. In terms of application to protein modification, the first method is currently applicable to proteins or long-chain peptides, while the second is limited to peptides with up to 10 residues. Therefore, this review focuses only on the first method through electrode oxidation. However, it is worth noting that in this approach, the modified residues themselves may also undergo electrode oxidation as the reaction progresses.