The mechanism of the alkylperoxo O–O bond cleavage of low-spin iron(III)–alkylperoxo species has been well established in nonheme iron models. In contrast, the alkylperoxo O–O bond cleavage in nonheme high-spin iron(III)–alkylperoxo species binding an axial ligand has yet to be elucidated. Herein, we report the synthesis and characterization of mononuclear nonheme high-spin iron(III)–alkylperoxo complexes each bearing an N-tetramethylated 13-membered macrocyclic ligand (13-TMC), [FeIII(OOC(CH3)3)- (13-TMC)]2+ and [FeIII(OOC(CH3)2C6H5)(13-TMC)]2+. The high-spin iron(III)–alkylperoxo complexes were converted to an iron(IV)–oxo complex at a fast rate upon addition of thiocyanate (NCS) via the formation of a short-lived intermediate. This intermediate was identified as a high-spin iron(III)– alkylperoxo complex binding a thiocyanate ion as an axial ligand by characterizing it with various spectroscopic methods and density functional theory (DFT) calculations. We have also provided strong evidence that conversion of the high-spin iron(III)–alkylperoxo complex to its corresponding iron(IV)–oxo complex occurs via O–O bond homolysis. Thus, we have concluded that the role of the axial ligand binding to a high-spin iron(III)–alkylperoxo complex is to facilitate the alkylperoxo O–O bond cleavage via the “push effect”, which has been well established in heme enzymes. To the best of our knowledge, the present study reports the first clear example showing the O–O bond homolysis of a high-spin iron(III)–alkylperoxo complex and the axial ligand effect on the alkylperoxo O–O bond cleavage in nonheme iron models.