The mechanism of tyrosine sensitizing the dimerization of thymine has been studied in this paper.,Deoxidized aqueous solutions of thymine (2.2×10,-3,~2×10,-2,M) containing tyrosine (1×10,-4,~1×10,-3,M)were irradiated at dry ice temperature using Co-60 γ-rays at 4×10,7,rads of absorbed dose, then a series of yields G,d, of the dimer were determined. In the range of studied tyrosine concentrations, it was found that the values G,d, varied with the concentrations of thymine and tyrosine and the values (1 - f,S,),3,G,d, all equal that obtained from the solution absenting tyrosine, however, the values (1-f,S,) 2K,A,G vary with the concentrations of thymine and tyrosine. Therefore it is evident that the mechanism of tyrosine sensitizing the dimerization of thymine is based on the energy transfer from excited state of tyrosine to aggregating state of thymine. The reaction dynamic process has been presented and the relationship between the dimer yield and the concentrations of thymine and tyrosine has been given as follows:,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42648181&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42648183&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42648198&type=,The values of G,2,/G,1, and (k,5,+k,7,)/k,6, have been given in this paper. The following conclusions can be drawn out:,1.owing to thymine with tyrosine being formed into mixed stacked aggregating state in frozen aqueous solution, the excited state energy of tyrosine can effectively transfer to aggregating state of thymine, then the yield of the dimer is incresed.,2.In the case of higher concentrations of thymine the sensitization of tyrosine to dimerization behaves via the mechanism of energy of excited single state of tyrosine transferring to aggregating state of thymine rather than the mechanism of the excited triplet state.,3.The increasing yield of the dimer varies with the concentrations of thymine and tyrosine.