Abstract: A method for the determination of the mefenacet residue in the paddy water and soil was established.The paddy water sample was filtrated and then analyzed by high performance liquid chromatography tandem mass spectrometry(HPLC-MS/MS).The linear range was 0.1-10 μ g/L with the linear correlation coefficient of 0.999 3.The limit of detection (LOD) was 0.03 μ g/L and the limit of quantification(LOQ) was 0.1 μ g/L.The average fortification recovery was 98.4%-103.2% with relative standard deviation(RSD) of 1.9%-3.4% when mefenacet was spiked at 0.1-10 μ g/L level in paddy water.The soil was extracted with the acetonitrile and cleaned up by the silica gel SPE column,then analyzed by high performance liquid chromatography with diode-array detector(HPLC-DAD).The linear range was 0.1-2 mg/L with the linear correlation coefficient of 0.998 5.The LOD was 0.006 mg/kg and LOQ was 0.02 mg/kg.The average recovery was 75.2%-86.1% with RSD 3.3%-7.5% when mefenacet was spiked at 0.02-1 mg/kg level in paddy soil.The method was used to detect the decline of mefenacet in the paddy water and soil in Beijing and Nanjing in 2009.The decline study of mefenacet in paddy water and soil suggested that the decline curves accorded with the first-order kinetics equation.Mefenacet declined fast in paddy water.The half-life of mefenacet in paddy water was 2.1 d(Beijing) and 1.6 d(Nanjing),respectively.The rate of decline in the paddy soil has an evident difference,and mefenacet declined slower in the paddy soil than in the paddy water.The half-life of mefenacet in paddy soil was 12.3 d(Beijing) and 3.7 d(Nanjing),respectively.