We herein describe the development of an LC-MS method for simultaneous determination of astilbin and 3'-O-methylastilbin in rat plasma. A simple liquid-liquid extraction procedure was followed by injection of the extracts on to a Shim-pack C(18) column (150 mm x 2.0 mm I.D., 5 microm) with gradient elution and detection in negative ionization mode. Initially, the method was validated regarding linearity, accuracy and precision. The correlation coefficients of all the calibration curves showed good linearity (r> 0.999) within test ranges, and the relative deviation was less than 10% for intra- and inter-day assays. Besides, this method was also validated for its stability, extraction efficiency, matrix effect and so on. Finally, this proposed method was successfully applied to rat pharmacokinetic study and yielded the most comprehensive data on systemic exposure of them to date.