In this paper the mecharism for radiation cleavage of thymine dimer in aqueous solution at room temperature has been studied. The results indicate that thymine dimer splitting by radiation is due to the reactions of hydroxyl radicals and hydrated electrons with the dimer and 0.5 thymine molecule is given for each hydroxyl radical and 1.5 thymine molecules for each reacted hydrated electron their rate constants are 2.3×10,9, dm,3,mol,-1,s,-1, and ≃1.0×10,10,dm,3,mol,-1,S,-1, respectively.