A method was developed for the determination of chloramphenicol,thiamphenicol and florfenicol residues in feedstuff by isotope dillution high performance liquid chromatography-tandem mass spectrometry.After homogenization,the samples were extracted with ethyl acetate.After centrifugation,the extract was evaporated to dryness with nitrogen at 45 ℃,and redissolved with 4 mL ultrapure water.The solution was defatted with hexane and cleaned up by C18 solid phase extraction cartridge.The targeted compounds were seperated on an Agilent Eclipse C18(150 mm×4.6 mm，5 μm) column by using methanol-0.1% formic acid as mobile phase.Identification and detection were achieved by electrospray ionization(ESI) under negative mode using multiple reaction monitoring(MRM) mode.The detection limits for the target compounds were all 0.5 μg/L and the quantitation limits were all 1.0 μg/L.The linear range of the target compounds ranged from 0.5 μg/L to 10.0 μg/L with correlation coefficients not less than 0999 0.Recoveries of three compounds were in the range of 78%-107% at the spiked level of 0.5-2.0 μg/kg,with relative standard deviations(RSDs) less than 10%.