A liquid chromatography-tandem mass spectrometry method was developed and validated for simultaneous determination of disulfoton and five of its oxidative metabolites (disulfoton-sulfoxide, disulfoton-sulfone, demeton-S, demeton-S-sulfoxide and demeton-S-sulfone) in human whole blood and urine. Extraction was undertaken using a QuEChERS method, which is commonly used in food analysis. D10-Disulfoton was used as the internal standard. Separation was carried out using a CAPCELL-PAK MG II column (35×2.0mm i.d., 5μm, Shiseido) with a mobile phase of 10mmol/L ammonium formate and methanol. This method was applied in an autopsy case, and disulfoton and its oxidative metabolites were successfully detected in both blood and urine. The concentrations of disulfoton in the blood and urine were 360 and 23.8ng/mL, respectively. There was a relatively low concentration of demeton-S in both the blood (4.0ng/mL) and urine (45.7ng/mL). To date, there have been no reported cases of detection of demeton-S in human samples.