Abstract: A method of determination of 2,4-D-butyl ester in Chinese Leek by GC/MS is established. 2,4-D-butyl ester in Chinese Leek was extracted with acetonitrile by ultrasonic wave. The extract was warmed with water at 80 ℃ and blowed with nitrogen gas until it got near to dryness. Then it was dissolved in acetone and cleaned-up by florisil-charcoal-alumina glass column. Retention time and specific ions were used to identify 2,4-D-butyl ester. The selective fragments were m/z 162, 175,185, 276, respectively. The calibration curve of 2,4-D-butyl ester was linear (correlatiom coefficient: 0.999 9) within the range of 0.002-1.0 μg/mL. The recoveries (spiking at 0.01 μg/mL and 1.0 μg/mL ) were 88.2%, 94.3%, respectively. The relative standard deviations (RSD) of the method was <10% for each case, and the limit of detection (LOD), defined in termes of 3 times baseline noise was 0.02 μg/kg. The method is suitable for determining 2,4-D-butyl ester in Chinese Leek and other vegetables and fruits.