In the present study, by using IPCC-MS3 (GL Sciences Inc. Tokyo, Japan) as the counter-ion in the mobile phase, we established a simple, quick method of analysis that separated and quantified paraquat and diquat on an ODS column by introducing the deproteinized serum sample directly into HPLC. The calibration curve of paraquat and diquat detected at UV 290 nm showed good linearity when the concentration of the injected sample was in the range 0.1-10.0 microg/ml. The detection limit was 0.05 microg/ml, and the mean recoveries (n=5) added 1.0 microg/ml each of paraquat and diquat to standard serum were 87.5% and 89.1%, respectively, while the RSD were 4.52% and 3.85%. All of these were good results, and the time taken for one analysis was less than 30 min. As a result of employing this analytical method for the analyses in four cases of acute poisoning, it was possible to decide promptly on treatment approaches for all of the present cases.