Abstract: The paper has described the rapid conversion of a low absorbing thymine dimer in the frozen aqueous solution of thymine by UY radiation. Thymine dimer has been isolated and identified and its properties of photochemistry and photobiology have been studied. The experiment results show that the staibility of thymine dimer is influenced by Ag⁺, Fe²⁺, Cu²⁺ and EB dye and the transformation between thymine dimer and thymine depends on the type and the concentration of metal ions.