Abstract: UHPLC-MS/MS was applied to the determination of arsanilic acid and roxarsone in feed. The crushed and screened sample was extracted ultrasonically for 30 min with acetonitrile (1+1) solution. After centrifugation, the supernatant was purified with methylene chloride. The water layer (upper level) was filtered on 0.22 μm filtering membrane of water system. Thermo HYPERCARB chromatographic column was used as stationary phase, and the mixture of 0.05% (φ) formic acid solution and acetonitrile 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 arsanilic acid and roxarsone were kept in the same range of 0.05-10.0 mg·L^-1, with detection limits (3S/N) in the range of 0.01-0.05 mg·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 82.0%-106%, with RSDs (n=6) of determination values in the range of 1.0%-7.9%.