Abstract: Simvastatin in human plasma was determined by LC-MS/MS. After protein precipitation the analyte and internal standard were separated on a Ultimate XB-C₁₈（2.1 m×100 mm，3μm） using the mobile phase of V(methanol)∶ V (acetonitrile)∶V(2.5 mmol /L ammonium acetate) =45∶50∶5 at a flow rate of 0.3 mL/min . Detection was carried out by electrospray positive ionizaion mass spectrometry in the SRM mode. The assay is linear over the range 0.1～20 μg/L with a lower limit of quantitation of 0.1 μg/L. Intra-day and inter-day precision are both less than 15%, respectively. The recoveries of Simvastatin are 71.96%～78.44%, and stabilities are good. The results show that it is a rapid, sensitive, selective and reliable method for the determination of Simvastatin in human plasma. The assay can be applied for the determination of Simvastatin in human plasma and the study on pharmacokinetics.