In this study, an ultra-performance liquid chromatography-electrospray ionization-mass spectrometry (UPLC-ESI-MS/MS) method was established for simultaneous determination of the 25 marker components, including chlorogenic acid, gallic acid, oxypaeoniflorin, homogentisic acid, methyl gallate, caffeic acid, 3,4-dihydroxybenzaldehyde, paeoniflorin, albiflorin, liquiritin, nodakenin, ferulic acid, ginsenoside Rg1, liquiritigenin, coumarin, cinnamic acid, benzoylpaeoniflorin, ginsenoside Rb1, cinnamaldehyde, paeonol, glycyrrhizin, 6-gingerol, evodiamine, rutecarpine, and spicatoside A in traditional Korean formula, Onkyung-tang. All analytes were separated on a Waters Acquity UPLC BEH $C_{18}$ analytical column ($2.1{\times}100mm$, $1.7{\mu}m$) at $45^{\circ}C$ using a mobile phase of 0.1% (v/v) formic acid in water and acetonitrile with gradient elution. The MS analysis was carried out using a Waters ACQUITY TQD LC-MS/MS coupled with an electrospray ionization (ESI) source in the positive and negative modes. The flow rate and injection volume were 0.3 mL/min and $2.0{\mu}L$, respectively. The correlation coefficient of all analytes in the test ranges was greater than 0.98. The limits of detection and quantification values of the 25 marker compounds were in the ranges 0.03-19.43 and 0.09-58.29 ng/mL, respectively. As a result, methyl gallate, 3,4-dihydroxybenzaldehyde, evodiamine, and rutecarpine were not detected in this sample and the concentrations of the 21 compounds except for the above 4 compounds were $33.09-3,496.32{\mu}g/g$ in Onkyung-tang decoction. Among these compounds, paeonol was detected at the highest amount as a $3,496.32{\mu}g/g$.