A highly selective and sensitive liquid chromatographic-tandem mass spectrometric (LC-MS-MS) method was developed and validated to determine daidzein and its main metabolite daidzein glucuronide in rat plasma. The analytes and internal standard genistein were extracted from plasma samples by n-hexane-diethyl ether (1:4, v/v), and separated on a C18 column. The mobile phase consisted of acetonitrile-water-formic acid (80 : 20: 1, v/v/v). Detection was performed on a triple quadrupole tandem mass spectrometer by selected reaction monitoring (SRM) mode via atmospheric pressure chemical ionization (APCI) source. The method has a limit of quantification of 0.24 ng/ml. The linear calibration curves were obtained in the concentration range of 0.24-1000 ng/ml. The intra- and inter-day precisions were lower than 13.2% in terms of % RSD. The accuracy ranged from -0.5% to 2.4% in terms of % RE (relative error). This method was successfully applied to the determination of plasma concentration of daidzein and its main metabolite daidzein glucuronide in rats after an oral administration of 20 mg/kg daidzein.