Epoxide is an important class of compounds, and is frequently formed in alkene oxygenation reactions. Detailed mechanism for the epoxide formation in reactions of alkene oxygenation via electron transfer is not known, however. In order to understand mechanism for the epoxide formation, we have performed a mechanistic study with DFT B3LYP/6-31G(d) level of theory for aliphatic alkenes. We have found that dimerization of peroxy radical cation, which gives the corresponding tetraoxide dication, followed by elimination of molecular oxygen, may be a major pathway for epoxide formation. We have also found that diradical that may be formed from peroxy radical cation via one electron reduction undergoes similar reaction to afford epoxide. Comparison of these results with those of electrochemical oxygenation of an alkene would also be presented.