A　density　functional　theoretical　(DFT)　method　was　applied　to　understand　the　effects　of　the　substituent　on　dioxygen　activation　by　a　series　of　substituted　iron　tetraphenylporphyrins　[FeT(o/p-R)　PP　(o=ortho-substituted,　p=para-substituted;　R=-H,　-Cl,　-NO2,　-CH3,　-OCH3)].　The　ground　states　(GS)　of　the　dioxygen　adducts　of　the　substituted　iron　tetraphenylporphyrins　[FeT(o/p-R)PPO2]　were　determined　at　the　B3LYP/6-31G(d)　level　without　any　symmetry　constraints.　Binding　energy　calculations　indicated　that　the　presence　of　electron-withdrawing　substituents　at　the　para　position　favors　O-2　binding.　Calculations　of　the　O-O　bond　length　of　the　adsorbed　dioxygen　revealed　that　the　influence　of　the　p-substituent　on　the　activation　of　dioxygen　decreases　in　the　order　p-CH3＞p-Cl＞p-OCH3＞-H＞p-NO2,　while　the　influence　of　the　o-substituent　decreases　in　the　order　o-NO2＞o-CH3＞o-Cl＞o-OCH3＞-H.　The　low-lying　excited　states　(LLES)　of　the　FeT(o/p-R)PPO2　adducts　suggest　that　the　ability　to　activate　dioxygen　decreases　in　the　order　o-CH3＞o-OCH3=-H＞o-NO2＞o-Cl　for　o-substituents　and　p-CH3＞p-Cl＞-H＞p-NO2＞p-OCH3　for　p-substituents.　NBO　charge　population　analysis　and　spin　density　analysis　showed　that　substitution　caused　more　beta-electrons　to　be　transferred　from　the　iron　tetraphenylporphyrin　to　the　dioxygen,　which　enhanced　dioxygen　activation.　Spin　density　analysis　confirmed　that　the　beta-electron　population　at　the　adsorbed　dioxygen　is　an　accurate　indicator　of　the　degree　of　dioxygen　activation.　The　trend　observed　in　porphyrin　catalytic　activity　as　the　substituent　on　the　dioxygen　adduct　was　varied　is　consistent　with　the　trend　in　the　binding　energy.　It　is　clear　that　substituents　at　the　ortho　and　para　positions　in　these　dioxygen　adducts　play　different　roles　in　dioxygen　activation.