Abstract: LC-MS/MS was applied to the determination of residual amounts of chlormequat and diquat in grains. The sample was extracted with a mixture of methanol, water and acetic acid, and purified by HLB-SPE column. The solution was used for LC-MS/MS determination, in which the Water ACOUITY UPLC BEH C18 column was used as stationary phase, and a mixture of 0.1% (φ) formic acid and acetonitrile (20+80) was used as mobile phase; positive electrospray ionization was used in the detection. Linearity ranges of chlormequat and diquat were found same between 5.00-500 μg·L-1 with detection limits (3S/N) of 2.5 μg·kg-1 for chlormequat and 5.0 μg·kg-1 for diquat. Values of recovery found by standard addition method were in the ranges of 77.7%-93.1% for chlormequat and 73.9%-95.3% for diquat, and values of RSD′s (n=7) were less then 15%.