Abstract: The photolysis of rich d -trans- allethrin , Es-biothrin , d -allethrin and hydrolysis of the three pyrethroids in water varying in temperature and pH were studied in the laboratory. Results of the test show that the photolysis of all the three pyrethroids in aqueous solutions under a 500 W xenon lamp fitted to the first order reaction dynamic equation, and the photolytic half-life values of rich d -trans- allethrin , Es-biothrin , d -allethrin in water were 1.84,1.37 and 1.51 h, respectively. The results also show that the hydrolytic rates of the three pyrethroids in the buffer solutions with pH being 5 and temperature at 25 ℃ were very low, with half-life longer than 162 d. With the temperature and the pH value rising, the hydrolytic rate was accelerated. The hydrolytic half-life values of rich d -trans- allethrin, Es-biothrin and d -allethrin in the buffer solutions with pH being 9 and temperature at 50 ℃ were 0.21,0.22 and 0.18 d, respectively. The experiment also shows that the three pyrethroids differed slightly in photolytic and hydrolytic rates in spite of their difference in isomer content.The mechanism of the hydrolysis was also discussed.