In this paper, we have investigated the effect of the radical and cation-anion scavenges on the dimerization of thymine in frozen aqueous solution induced by r-rays. It was proved that the indirect effect of OH radical produced by water rediolysis is not the cause of forming the thymine dimer linked by the cyclobutane ring. The cation-anion recombination is not the way of forming the dimer too. From the research of excitation transfer we have obtained the evidence that thymine dimer linked by the cyclobutane ring is caused by the excited state of thymine. At the same time, We have found that the thymine dimer A can also be formed at 268 K.