Abstract: The study established a LC-MS/MS method for the simultaneous determination of two active diterpenoids:kirenol and ent-16β, 17-dihydroxy-kauran-19-oic acid (DHKA) from Herba Siegesbeckiae in rat plasma using osthole as an internal standard (IS). Plasma sample pretreatment involved a one-step liquid-liquid extraction with ethyl acetate. Chromatographic separation was performed on a Waters Symmetry C₁₈ column (2.1 mm×100 mm, 3.5 μm) with isocratic elution using methanol-5 mmol·L^-1 aqueous ammonium acetate (80:20) as mobile phase at a flow rate of 0.2 mL·min^-1. The detection was performed on a triple quadrupole tandem mass spectrometer in multiple reaction monitoring (MRM) mode under positive and negative electrospray ionization. Quantification was performed using SRM of the transitions m/z 356.4→321.4 for kirenol, m/z 335.3→335.3 for DHKA, and m/z 245.1→188.9 for the IS, respectively. The calibration curves were linear over the range of 50.0-25000 ng·mL^-1 for kirenol, 25.0-12500 ng·mL^-1 for DHKA. The extraction recoveries of two analytes and IS were more than 85%. The intra- and inter-day precision (relative standard deviation) values were less than 13.9% and accuracy (relative error) was from -10.7% to 10.3% at four quality control levels. The pharmacokinetic parameters of different medication administration teams were analyzed with SPSS statistics 13.0 software. The validated method was successfully applied to a comparative pharmacokinetic study of the two diterpenoids in rat plasma after intragastric administration of kirenol, DHKA and Herba Siegesbeckiae extract. Kirenol appeared to be both absorbed and eliminated fast in vivo, and DHKA absorbed fast but eliminated slowly in vivo. And there were obvious differences between the pharmacokinetic behaviors after oral administration of Herba Siegesbeckiae extract compared with single substances. Compared with the value after oral administration of kirenol, the extract might inhibit the absorption and postpone the elimination of kirenol in rats after administration of the extract. For DHKA, the absorption rate of DHKA increased rapidly after administration of the extract. This work can provide some experimental basis for the clinical use of Herba Siegesbeckiae. The method is simple, rapid and sensitive, which is suitable for pharmacokinetics study of the two diterpenoids from Herba Siegesbeckiae in rats.