Abstract: A quantitative and qualitative method of ultra-performance liquid chromatography coupled to triple quadruple tandem mass spectrometry （UPLCTM-MS/MS） has been developed for the rapid and sensitive determination of the protein binding of sulfamonomethoxine(SMM) with common carp (Cyprinus carpio) plasma. After extraction with acetonitrile and defatted with n-hexane, the plasma extract was performed with the C18 column（BEH C18 1.7 μm, 2.1 mm × 50 mm）, and then separated by gradient elution using ammonium acetate/formic acid solution and acetonitrile. Mass spectrometric determination was carried out with an electrospray ionization interface（ESI）in positive ion mode with multiple reactions monitoring mode（MRM）. The calibration curve ranging from 0.5 to 200 ng/mL in fluent phase solvent was used to establish instrument response, with relative standard coefficient beyond 0.998. The method was validated by analysis of nine replicate plasma samples fortified with three kinds of concentration levels of 2 μg/L，20 μg/L and 100 μg/L SMM, respectively. Depending on the biological matrix and analyte，relative standard deviations were below 15 %，with average recovery rates around 76.3 % to 90.3 % in spiked samples and the limit of detection （LOD） and quantification （LOQ） for the analyte were 0.25 μg/kg and 0.5 μg/kg using 1.0mL sample， respectively，and the precision and recovery were all satisfying. Finally, the method was successfully used to determinate the protein binding of common carp plasma with SMM under 20℃ by equilibrium dialysis method, and the equilibrium time was 40 h and the rate of protein binding was 20.9%.[Chinese Fishery Quality and Standards, 2012, 2（4）：00-00]