A selective and sensitive liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed for the simultaneous quantitative determination of 1,5-dicaffeoylquinic acid (1,5-DCQA) and 1-O-acetylbritannilactone (1-O-ABL) in rat plasma. Chromatographic separation was performed on a Zorbax Eclipse XDB-C column using isocratic mobile phase consisting of methanol-water-formic acid (70:30:0.1, v/v/v) at a flow rate of 0.25 mL/min. The detection was achieved using a triple-quadrupole tandem MS in selected reaction monitoring mode. The calibration curves of all analytes in plasma showed good linearity over the concentration ranges of 0.850-213 ng/mL for 1,5-DCQA, and 0.520-130 ng/mL for 1-O-ABL, respectively. The extraction recoveries were ≥78.5%, and the matrix effect ranged from 91.4 to 102.7% in all the plasma samples. The method was successfully applied for the pharmacokinetic study of the two active components in the collected plasma following oral administration of Inula britannica extract in rats.