The mechamism of releasing inorganic phosphate from Deoxythymidine-5'-monophosphate(dTMP) and radiation protection on releasing inorganic phosphat from dTMP by Histidine have been inverstigated.The release of inorganic phosphate from dTMP results mainly from the acting of OH radical with H atom at position ,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42767396&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42767476&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42767460&type=,. It was calculated that rate constant for reaction of OH radical with position ,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42767468&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42767472&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42767485&type=, is (1.7±0.1)×10,8,M,-1,S,-1, and the rate constant for reaction of OH radical with dTMP is (5.0±0.2)×10,9,M,-1,S,-1,.,Because the OH radical was scavenged by Histidine effectively, the radiation protection from releasing inorganic phosphate from dTMP by Histidine was obvious. It may be seen from viscosity change of DNA after irradiation that Histidine exhibits marked protection from denaturation and degradation of DNA.