The in vivo effects of traditional herbal medicines are generally mediated by multiple bioactive components. The main constituents of Polygonum orientale L. are flavonoids such as orientin, vitexin, cynaroside, and quercitrin. The aim of this study was to develop and validate a method for characterizing these flavonoids, in order to better understand the pharmacokinetics and pharmacodynamics of P. orientale L. We used ultra-performance liquid chromatography-electrospray ionization-tandem mass spectrometry (UPLC-ESI-MS/MS) to analyze the flavonoids. After precipitation of the proteins with methanol, the flavonoids were separated on a BEH C18 column (50mm×2.1mm, i.d., 1.7μm) by using an elution gradient of acetonitrile. Flavonoid content was determined using the multiple reaction monitoring (MRM) mode at m/z 449.2→329.2 for orientin, m/z 433.2→313.0 for vitexin, m/z 449.2→287.1 for cynaroside, m/z 449.2→303.4 for quercitrin, and m/z 417.0→267.0 for the internal standard, puerarin. Pharmacokinetics was assessed after intravenous administration of P. orientale L. extracts (POE) in Beagle dogs at a dose of 22, 44, or 88mg/kg. Analysis of the standard curves by linear regression revealed high linearity over a 243-fold dynamic range for the four flavonoids (the lower limit of quantitation values were 4-21ng/mL). The relative standard deviations of intra- and inter-day measurements were less than 15.1%, and the method was accurate to within -8.7% to 7.2%; the extraction recoveries from dog plasma were 70.6-89.3%, 69.8-88.7%, 72.5-85.7%, and 71.0-79.1% for orientin, vitexin, cynaroside, and quercitrin, respectively. Our results suggest non-linear pharmacokinetic characteristics with rapid clearance of the flavonoids. In conclusion, UPLC-ESI-MS/MS is a rapid and sensitive method for simultaneous quantification of multiple flavonoids from POE in dog plasma and is suitable for pharmacokinetic studies of herbal medicines.