Abstract: It was found that in a B-R buffer solution of pH 6.5,the absorption of methylene blue at its absorption maxima of 666 nm was remarkably decreased upon the addition of DNA,and linear relationship was obtained between the magnitude of decrease in absorbance (ΔA) and concentration of DNA in the range within 10.0 mg·L-1,with correlation coefficient of 0.999 8 and detection limit of 0.10 mg·L-1.The proposed method was applied to the determination of DNA in 3 simulated samples based on proteinase,trypsin and bacteriolysin,giving results of recovery in the range of 98.4%-101.0% and values of RSD′s (n=11) in the range of 1.1% to 3.1%.Optimum conditions (i.e.effects of concentration of methylene blue solution,temperature of reaction solution,acidity for the reaction and time required for the reaction) were tested and determined.A brief discussion on the reaction mechanism was also given.