Fenton reaction is the primary source which generates hydroxyl radical in vivo.It has been reported that manganous substitute for iron in Fenton reaction system to evolve hydroxyl radical（Fenton like reaction）.In this research

electron paramagnetic resonance（ESR） spectroscopy was used to measure the intensity of hydroxyl radical which generated from Fenton reaction and Fenton like reaction.It was shown that the intensity of hydroxyl radical generated from Fenton like reaction was merely 5% compared with that from Fenton reaction at the beginning of the reaction.HPLC with fluorescence detection（excitation 277 nm and emission 306 nm） was used to detect the generation of hydroxyl radical in Fenton reaction to which micro amount of manganous was added.L-phenylalanine

which could be converted to highly fluorescent L-tyrosine in the presence of hydroxyl radical

was used as the probe.It was shown that the generation of hydroxyl radical was obviously promoted by manganous.In addition

DNA strand breaks were also used to verify above results.The results suggested that hydroxyl radical promoted by manganous in Fenton reaction might be an important reason which resulted in oxidative stress in vivo induced by manganous.