Abstract: Residual amounts of malachite green (MG) and leuco malachite green (LMG) in aquatic products were determined by ultra-high performance liquid chromatography (UHPLC) hyphenated with tandem mass spectrometry (MS/MS).The compounds,MG and LMG,were extracted from the homogenized tissue sample with acetonitrile in an acetate buffer medium of pH 4.5,separated and purified by liquid-liquid partition and acidic alumina and sodium chloride,as well as acidic alumina and anhydrous sodium sulfate in succession.The clear supernatant liquid after fitration with 0.2 μm filtering membrane was ready for UHPLC-MS/MS analysis.Determinations were performed by using electrospray ionization (ESI+) and multi-reaction monitoring (MRM) modes.Internal standards of D5-MG and D6-LMG were used in the quantification.Under optimized conditions of sample pretreatment and operation of instruments,matrix effect on the MS determination was reduced rationally.Determination (S/N=10) for MG and LMG were found to be 0.19 and 0.10 μg·kg-1 respectively.Tests for recovery were made by addition of standards in the concentration range of 0.5 to 5.0 μg·kg-1,and results obtained were ranged from 89.4% to 110%.Values of RSD′s (n=6) were found in the range of 5.9%-9.8%.