Abstract: The hydrolysis and photolysis of monosulfuron-ester in water were investigated by high performance liquid chromatography in laboratory conditions.The hydrolysis half-life of monosulfuron-ester were 13.1,192 and 347 d in pH 5,pH 7 and pH 9 buffer solution respectively and were classified as easily or difficultly hydrolysis type under 25 ℃.In the condition of 50 ℃,the half-life of hydrolysis of monosulfuron-ester were 19.6 h and 4.6,7.1 d in three different buffer solutions correspondingly and were classified as easily hydrolysis type.The hydrolytic rate of monosulfuron-ester increased with higher temperature and temperature coefficients were 32.7-48.9.Monosulfuron-ester is degraded most rapidly in acid buffer and most tardily in alkali buffer.There were significant positive correlations between both of the activation energy and activation entropy with pH value of buffer solution.The photolysis half-life of monosulfuron-ester was 4.9 h and classified as easily photolysis under the condition of 25 ℃,3 620 lx of light intensity and 71.1 μW/cm2 of UV intensity.