Reactive oxygen species (ROS) cause various types of damage to many biomolecules, including lipids, proteins, DNA and so on. ROS may contribute to various diseases. Much of the damage caused by superoxide anion radical and hydrogen peroxide in vivo is thought to be due to their conversion into more reactive species, including hydroxyl radical, and probable ferryl iron species. The experimental material and theoretic result have shown that in addition to generation of hydroxyl radical, the ferryl iron species have been proposed to be involved when iron salts are mixed with hydrogen peroxide. The reactive intermediate of Fenton reagents is a nucleophilic adduct. The products profiles and reactivity for Fenton reagents are different from those for free hydroxyl radical and depend upon the transition metal, the ligand, the solvent matrix and substrate.