Abstract: A rapid, sensitive and reliable high performance liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed for the analysis of DEC in eggs, and the method has been successfully applied to the depletion study of DEC. The eight healthy laying hens fed medicated feed with decoquinate concentration of 27.2 mg/kg for 10 consecutive days. The egg samples were collected on days 1, 2, 3, 5, 8, 11 after the last dose and homogenized. The compound was extracted by acetonitrile-ethyl acetate using liquid-liquid extraction (LLE), and purified with OasisTM HLB Solid Phase extraction cartridge. Chromatographic separation was performed on an XTerra C18 reversed-phase column with a mobile-phase of water containing 0.1% formic acid and acetonitrile. The analyte was detected by tandem quadrupole mass spectrometry after positive electrospray ionization by multiple reaction monitoring (MRM). The results demonstrated that the method has high sensitivity with the LOD of 0.2μg/kg and the LOQ of 0.8μg/kg. The calibration curve showed good linearity with the concentrations of 0.8-240μg/kg with the correlation coefficient (R2>0.993). The recovery ranges from 90.7% to 101.8%, and RSD<11.4%. The result of depletion profile of DEC showed that the highest residue concentration of DEC was attained 3 days post-treatment. The residue concentration of DEC decreased to 20μg/kg which was the allowable concentration level of European Commission at 8 days after the last dose. The result showed that the method was rapid and reliable for application in practice.