Abstract: OBJECTIVE To establish a ultra-high performance liquid chromatography-tandem mass spectrometry method (UPLC-MS/MS) to determine the concentration of a new fatty acid synthesis inhibitor 4k in rat plasma, and study on its pharmacokinetic characteristics. Methods Twelve SD rats were randomly divided into two groups, and 4k was given by single intravenous injection and intragastric administration respectively. Using triclosan as the internal standard, the UPLC-MS/MS method was established and verified to determine the concentration of 4k in rat plasma at different time points, and the pharmacokinetic parameters were calculated by DAS 2. 0. Results The main pharmacokinetics parameters of SD rats were as follows:T_1/2 (0.54±0.39)h, T_max 0.033 h, C_max (2 730.72±803.13)ng·mL^-1, AUC_0-∞ (577.72±174.58)ng·mL^-1·h, Vd (1 241.17±657.98)mL·kg^-1, CL (1 882.67±610.03)mL·kg^-1·h^-1, MRT_0-∞ (0.42±0.19)h. After intragastric administration, the blood drug concentration in rats was lower than the lower limit of quantification, so the pharmacokinetic parameters could not be calculated. Conclusion This method is simple, accurate, rapid and sensitive, and suitable for the determination of 4k concentration in rat plasma and its pharmacokinetic study.