The mechanism of the dimerization of thymine in frozen aqueous solutions induced by γ-rays was described. During the study of dynamic mechanism of the dimerization, it was found that the thymine dimer linked by a cyclobutyl ring is formed by way of excited triplet state mechanism at lower concentration of thymine but by way of mechanisms of excited triplet state and excited aggregating state at higher concentration of thymine. These mechanisms agree with following dynamic equations respectively:,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42489724&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42489726&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42489725&type=,In this paper, some dynamic parameters are given and the sources to form excited state of thymine are discussed.