In this paper, we study the reaction mechanism of 5'-GMP releasing the inorganic phosphate (Pi) induced by γ-ray. It is proved that hydroxyl radical produced by water radiolysis is the main active species which induct 5'-GMP releasing inorganic phosphate and H atom has also some contribution for inorganic phosphate release from 5'-GMP. It is measured by the competition kinetic method that the rate constants of the reaction of OH radical with 5'-GMP ,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42648434&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42648435&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42648401&type=, and 5'-GMP are 1.2×10,8,(mol/L) ,-1,S,-1, and 5.1×10,9,(mol/L) ,-1,S,-1, respectively.