High valent Fe^IVO species has been proposed to play a key role in the catalytic cycle of non-heme mono-iron enzymes. Extensive studies have been performed to reveal the catalytic mechanism of the enzyme and to reproduce its catalytic reactivity using biomimetic Fe^IVO complexes with synthetic ligands. In this study, the mechanism of C-H activation has been investigated at an atomic detail for four synthetic complexes with an Oh symmetry and two with a Tbp symmetry including a TauD-J model (see below), based on the DFT potential energy surfaces of both S =1 and S =2 spin states. This study demonstrated that all the high spin Fe^IVO complexes, irrespective of their symmetry (Tbp or Oh), utilize a species like an Fe^III-oxyl radical for C-H activation, which is formed en route to the TS before the actual HAT occurs. However, no such species is formed in the intermediate spin state (S = 1). The reason why the reactivity of high spin complexes depends on ligand symmetry even with nearly identical mechanism will be discussed.