Abstract: Foodstuff sample was extracted with acetonitrile-water (1+1) solution, and the extract obtained was purified by passing through gel permeation chromatographic (GPC) and solid phase extraction (SPE) columns. The eluate obtained was determined and identified by LC-MS/MS with external standard method for quantification. Mixtures of acetonitrile and HCOOH-H2O (0.1+99.9) solutions of various ratios were used as mobile phase in gradient elution and electrospray ionization source with negative ionization mode as well as multiple reaction monitoring mode were used in the MS/MS analysis. Linear relationship between values of peak area and mass concentration of mesotrione was kept in the range of 0.01-0.2 mg·L-1. Tests for recovery were made by addition of standard mesotrione solution at 3 different concentration levels to 4 kinds of foodstuff as matrixes, values of recovery found were in the range of 73.2%-100.6%, with values of RSD′s (n=10) in the range of 4.1%-11%.