Abstract: OBJECTIVE To establish an ultra-high performance liquid chromatography method for the determination of duloxetine in human plasma and to apply this method for therapeutic drug monitoring (TDM) of duloxetine. METHODS The LC-MS/MS was applied for determination of duloxetine plasma concentration with glibenclamide as internal standard and Phenomenex Synergi Hydro-RP(2.00 mm×50 mm, 0.004 mm) as column. The mobile phases were consisted of 0.1% formic acid in acetonitrile and 0.1% formic acid in water and were pumped at a flow rate of 0.6 mL·min^-1. Quantitative analysis was conducted in the multiple reaction monitoring modes with product ion transitions of m/z 298.197/154.200 and m/z 494.200/369.100 for duloxetine and internal standard, respectively. RESULTS Chromatograms showed no endogenous interfereing peaks in the respective blank human plasma samples. The calibration was linear in the concentration range with 5-200 ng·mL^-1(r=0.999 3) and the relative recoveries were at 93.64%-105.3%. Inter-day and intra-day RSD were less than 10.0% and 8.2%, respectively. Both the extraction recovery matrix effect and the stability were validated for duloxetine in human plasma. The plasma concentration of duloxetine levels were significantly affected by age(P<0.05), but not by genter. CONCLUSION This method is simple, rapid, specific and precise for the determination of duloxetine in human plasma and it could be widely used for TDM. The plasma concentrations of duloxetine were variability, especially significantly affected by age. Rotine TDM of duloxetine may improve the treatment response.