An ultrasonic decomposition of synthetic wastewater containing methanidophos is reported. The effects of power intensity, irradiation time, solution temperature, initial PH, Fe2+ and H2O2 concentrations on the decomposition of methamidophos have been examined. Under the conditions of initial concentration of methamidophos 0. 5-5. 0×10-4 mol/L, PH = 2. 5-10. 8, 30℃, power intensity 80 W/cm2, Fe2+ concentration 50 mg/L in 2 h from the oxygen saturated sample solution 99 .2 % of methamidophos were decomposed according to the COD measurement results.