Abstract: HPLC-MS/MS was applied to the simultaneous determination of monepantel and monepantel sulfone in dairy products (milk powder or liquid milk). The milk powder sample (about 0.5 g) was vortexed to dissolve completely in 2 mL of water. The above milk powder sample solution or about 2 mL of liquid milk sample was extracted ultrasonically with saturated sodium chloride solution (5 mL) and 18 mL of acetonitrile. After concentration, the extract was purified with n-hexane. Inertsil C₈-3 chromatographic column was used as stationary phase, and the mixture of 0.5 mmol·L^-1 ammonium acetate solution and methanol mixed in different ratios was used as mobile phase in gradient elution. ESI^- and multi-reactions monitor mode were adopted in MS/MS. Linear relationships between values of peak area and mass concentration of monepantel and monepantel sulfone were kept in the same range within 5.0 μg·L^-1, with detection limits (3S/N) in the range of 0.008-0.018 μg·kg^-1. On the base of blank sample, test for recovery was made by standard addition method; values of recovery found were in the range of 79.5%-104%, with RSDs (n=6) in the range of 2.8%-5.9%.