Abstract: To establish a simple and sensitive LC-MS/MS method for simultaneous determination of the concentration for L-tryptophan(L-Try)and L-kynurenine(L-Kyn)in rat plasma. The changesin the process of the liver tumors formation may provide a basis for the diagnosis of liver cancer. 3-Nitro-L-tyrosine(3-NT)were added as the internal standard for the determination of two active substances and the chromatographic analysis was performed on a RRHD Eclipse Plus C₁₈ column(3. 0 mm×100 mm, 1. 8 μm). The mobile phase was composed of water and acetonitrile(containing 0. 1%formic acid)(90 ∶10)at a flow rate of 0. 25 mL/min, and the injection volume is 5 μL. Detection and quantification were performed by mass spectrometry in multiple reaction monitoring mode with m/z 205. 12→146. 10(L-Try), m/z 209. 09 →146. 10(L-Kyn), m/z 227. 09→181. 10(3-NT), respectively. The results show that thelinear ranges were 9. 670-9 670 ng/mL for L-Try, and 9. 973-9973 ng/mL for L-Kyn(r²≥0. 9990). The limit of quantitation were 9. 670 ng/mL for L-Try, and 9. 973 ng/mL for L-Kyn, respectively. The intra- and inter-day precisions were all less than15%; the recoveries ofthe two analytes were more than 81. 17% and severe matrix effect was not observed. The ratio of L-Try/L-Kyn determined by LC-MS/MS in rat plasma showed an overall downward trend, which could used effectively for the drug metabolism studies and researches on the action mechanism of medicine on liver cancer. A rapid, simple, sensitive and specific LC-MS/MS method has been successfully developed and could also be used effectively for the drug metabolism studies and researches on the action mechanism of medicine on liver cancer.