A sensitive, simple, and accurate method was developed for the determination and pharmacokinetic comparison of cinnamic acid in rat plasma after the administration of a Traditional Chinese Medicinal preparation, Di-Gu-Pi decoction, and pure cinnamic acid using RP-HPLC. Di-Gu-Pi was extracted with 5% aqueous sodium bicarbonate, which was followed by purification with ion exchange column chromatography. The plasma samples taken from rats were deproteinized with methanol. The reversed-phase(HPLC) system with a Diamonsil C_ 18 column and methanol-acetonitrile-water (8∶32∶60, volume ratio) (adjusted to pH=3.0 with glacial acetic acid) as the mobile phase was employed for the separation of cinnamic acid in the plasma samples. The detection was set at 272 nm and 3-(p-fluorophenyl)-2-propenoic acid was chosen as the internal standard. The calibration curve was linear in a range from 0.10 to 25.0 μg/mL (R 2 =0.9988, n=9). The precision was 3.42%—10.10%; the between-day precision was 2.84%—8.91%; the accuracy was-1.51%—1.26%; the mean recovery was 99.9%. The method was found to be sensitive, simple, accurate and appropriate for the determination of cinnamic acid. A sensitive, simple, and accurate method was developed for the determination and pharmacokinetic comparison of cinnamic acid in rat plasma after the administration of a Traditional Chinese Medicinal preparation, Di-Gu-Pi decoction, and pure cinnamic acid using RP-HPLC. Di- The plasma samples taken from rats were deproteinized with methanol. The plasma samples were taken from rats were deproteinized with methanol. The plasma samples were taken from rats with deaminated with methanol -acetonitrile-water (8:32:60, volume ratio) (adjusted to pH = 3.0 with glacial acetic acid) as the mobile phase was employed for the separation of cinnamic acid in the plasma samples. The detection was set at 272 nm and The calibration curve was linear in a range from 0.10 to 25.0 μg / mL (R 2 = 0.9988, n = 9). The precision was 3.42% -10.10%; the between-day precision was 2.84 % -8.91%; the accuracy was-1.51% -1.26%; the mean recovery was 99.9%. The method was found to be sensitive, simple, accurate and appropriate for the determination of cinnamic acid.