The interaction of allethrin and tetramethrin with single strand DNA (ssDNA)，double strand DNA, guanine and guanosine was studied by means of UV and fluorescence spectrophotometry. Pyrethroid Pesticide brought about a hypochromic effect on the UV absorption spectrum of DNA, but the maximal absorption wavelength of DNA did not shift obviously and the addition of varied concentration of DNA caused the fluorescence quenching of pesticides. The effect of ionic strength and KI on the system of pesticide-DNA and the effect of pesticide on the neutral red-DNA were investigated. The results have indicated that the electrostatic binding and the groove binding are the two major modes of the interaction between allethrin and DNA, while electrostatic binding mainly exists between tetramethrin and DNA. The spectra of UV and the fluorescence of Guanine and guanosine interacting with pesticides have proved that the interaction between them and pesticide is chiefly hydrogen bonds, and guanosine interacts more strongly with pesticides than guanine does.