An isotope dilution high performance liquid chromatography-tandem mass spectrometric（HPLC-MS/MS） method was established for the determination of biurea in flour and flour products.The biurea in certain matrices was extracted with ultrapure water,defatted with n-hexane when necessary,and then analyzed by HPLC-MS/MS.LC separation was performed on a Waters XBridge BEH Amide column（3.5 μm，4.6 mm×150 mm） using ultrapure water and acetonitrile as mobile phase by gradient elution.Identification was achieved by electrospray ionization（ESI+） in positive mode using multiple reaction monitoring.Quantification was performed by isotope labeled 13C2,15N2-biurea internal standard calibration.The calibration curve of biurea showed a good linearity in the range of 0.2-100 μg/L with a correlation coefficient of 0.999 9.The average recoveries of biurea in flour,steamed bun,bread,deep-fried dough stick and noodle matrices（six aliquots for each level） at three spiked levels ranged from 82.0% to 113.6%,with RSDs of 2.1%-9.3%.The LOQ of the methed was 20 μg/kg.The optimized method was successfully applied in the analysis of 136 real samples collected from local markets in Southern China，while biurea residues were found in 42（30.88%） samples,of which 43.7-564 μg/kg of biurea existed in 4 flours,70.8-9 840 μg/kg in 2 noodles,2 480 μg/kg in 1 deep-fried dough stick,27.4-8 730 μg/kg in 15 breads-crust and 64.3-18 100 μg/kg in 20 breads-interior.This method is applicable for the determination of biurea in flour and different flour products due to its operation convenience,high sensitivity and good accuracy.