A robust and validated method based on ultra-performance liquid chromatography coupled with tandem mass spectrometry (UPLC-MS/MS) has been developed for the simultaneous determination of remimazolam, which is a new chemical entity, and its major carboxylic acid metabolite (M1) in human plasma. Plasma samples were pre-purified by protein precipitation procedure and analyzed using an isocratic chromatographic separation over an Acquity UPLC CSH C18 column. The mobile phase consisted of acetonitrile-water containing 10mM ammonium formate and 0.1% formic acid at a flow rate of 0.4mLmin(-1). Positive electrospray ionization was employed as the ionization source in the multiple reaction monitoring (MRM) mode. The analysis time was about 1.5min. The method was fully validated over the concentration range of 0.5-1000ngmL(-1) for both analytes. The lower limit of quantification (LLOQ) was 0.5ngmL(-1). Inter- and intra-batch precision was less than 8.4% and the accuracy was within 88.8-107.0%. The mean extraction recoveries obtained from three concentrations of QC plasma samples were 96.8%, 98.7% and 98.6% for remimazolam, 98.7%, 99.8% and 101.5% for M1, respectively. Selectivity, matrix effect and stability were also validated. The method was applied to the pharmacokinetic study of remimazolam in Chinese healthy subjects.