A liquid chromatography-tandem mass spectrometric（LC-MS/MS） method was developed for the determination of tyrosine isomers（o-tyrosine and m-tyrosine） in protein-containing functional food.The sample was dissolved with 6 mol/L hydrochloric acid,staying overnight under 110 ℃ acid solution after full of nitrogen,then evaporated with a stream of nitrogen to near dryness and redissolved with water.The LC separation was performed on a Phenomenex Kinetex PFP column with 5 mmol/L ammonium acetate solution and methanol as mobile phases by gradient elution.The tyrosine isomers were determined by MS/MS in negative electrospray ionization（ESI） mode,and quantified by the matrix-matched（MRM） external standard method.Good separations were obtained between the tyrosine isomers and other impurity peaks .In 0.005-1.0 mg/L concentration range,the peak areas of tyrosine isomers against concentration showed good linear relationships with correlation coefficient（r2）more than 0.999.The average recoveries of tyrosine isomers in protein-containing function food at three spiked concentration levels ranged from 81.8% to 98.2%,with relative standard deviations（RSDs,n=6） less than 8.0%.The limit of quantitation（LOQs） were 5.0 μg/kg and 8.0 μg/kg,and the limits of detection（LODs） were 1.5 μg/kg and 2.4 μg/kg,respectively.This method was applied in real samples,and the result indicated that the radiation dose could be quantitatively evaluated by the linear correlation with coefficients of determination（r2） more than 0.98 in individual irradiated protein containing function food samples.The method could be applied in qualitative identification of tyrosine isomers in protein-containing functional food.