Complanatoside A is a flavonol glycoside isolated from Astragalus complanatus, and currently it is used as a quality control index for A. complanatus in the 2010 edition of the Chinese Pharmacopoeia. In Vitro: A simple and sensitive LC-MS/MS method is developed for the determination of complanatoside A in rat plasma over the range of 2.3–575 ng/mL. Complanatoside A is extracted from plasma by a protein precipitation procedure, separated by LC and detected by MS/MS in positive electrospray ionization mode. The lower limit of quantification is established at 2.3 ng/mL. Intra- and inter-day precisions (LLOQ, low-QC, med-QC and high-QC) are less than 7.9%, and accuracies are between 94.0 and 105.1%. Matrix effect is acceptable (97.9–103.0%) and extraction recovery is reproducible (88.5–94.4%). Complanatoside A is stable in the investigated conditions. The method is applied to the pharmacokinetics of complanatoside A in rats^[1].