Objective To establish an LC-MS method to determine ursodeoxycholic acid and chenodeoxycholic acid in rat plasma and to investigate pharmacokinetic properties of ursodeoxycholic acid and chenodeoxycholic acid after an oral administration of bear gall powder extract to rats.Methods Ursodeoxycholic acid and chenodeoxycholic acid and diclofenac (the internal standard, IS) were extracted by liquid-liquid extraction and separated on a Waters Atlantis T3 C18 column, with methanol and water containing 1 mmol·L-1 ammonium acetate and 0.01% formic acid (24:76) as the mobile phase.Quantitation was carried out by single reaction monitoring in neg. mode on a Shimadzu LC-MS2020 system.After the validation, the developed method was used to evaluate the pharmacokinetics of ursodeoxycholic acid and chenodeoxycholic acid in rats after an oral administration of 100 mg·kg-1 of bear gall powder extractResults The study showed a good linearity at 0.078-10.0 μg·mL-1 with a sensitivity of 0.078 μg·mL-1 as the limit of quantification.The precision, accuracy, stability and mean recoveries were satisfied and met the requirement of biol. sample measurement.The method was successfully applied in the pharmacokinetic study of ursodeoxycholic acid and chenodeoxycholic acid in rat plasma samples.Conclusion This LC-MS method was selective, sensitive and reproducible, and was successfully applied in the pharmacokinetic study of ursodeoxycholic acid and chenodeoxycholic acid in rats.