Through the introduction of dimethylamino (Me₂N) substituent at the pyridine ring of 2-((R)-2-[(R)-1-(pyridine-2-ylmethyl)pyrrolidin-2-yl]pyrrolidin-1-ylmethyl)pyridine (PDP) ligand, the non-heme Fe^II(^Me2NPDP)/H₂O₂/AcOH catalyst system was found to exhibit significant higher catalytic activity and enantioselectivity than the non-substituent one in the asymmetric epoxidation experiments. The mechanistic origin of the remarkable substituent effects in these oxidation reactions has not been well established. To ascertain the potent oxidant and the related reaction mechanism, a detailed DFT calculation was performed. Interestingly, a novel Fe(IV)-oxo ^Me2NPDP cation radical species, [(^Me2NPDP)^+·Fe^IV(O)(OAc)]²⁺ (^Me2N5), with about one spin spreading over the non-heme ^Me2NPDP ligand was formed via a carboxylic-acid-assisted O–O bond heterolysis, which is reminiscent of Compound I (an Fe(IV)(O)(porphyrin cation radical) species) in cytochrome P450 chemistry. ^Me2N5 is energetically comparable with the cyclic ferric peracetate species ^Me2N6, while in the pristine Fe(PDP) catalyst system, ^H6 is more stable than ^H5. Comparison of the activation energy for the ethylene epoxidation promoted by ^Me2N5 and ^Me2N6, ^Me2N5 is supposed as the true oxidant triggering the epoxidation of olefins. In addition, a systematic research on the substituent effects varied from the electron-donating substituent (dMM, the substituents at sites 3, 4, and 5 of the pyridine ring: methyl, methoxyl, and methyl) to the electron-withdrawing one (CF₃, 2,6-bis(trifluoromethyl)phenyl) on the electronic structure of the reaction intermediates has also been investigated. An alternative cyclic ferric peracetate complex is obtained, indicating that the substituents at the pyridine ring of PDP ligands have significant impacts on the electronic structure of the oxidants.