Polygala Radix (“Onji”) is one of an important crude drug used in Kampo-medicine. The method of quantification of the three characteristic constituents (tenuifolin, polygalaxanthone III, 3,6'-disinapoyl sucrose) were prescribed by a crude drug (“Onji”) in Pharmacopoeia of People’s Republic of China (ChP). We are considering a testing method. It was found that the quantitative value was high because it was quantified in a form including impurity in addition to Polygalaxanthone III. Therefore, we succeeded to find a method that can measure only Polygalaxanthone III by improving the quantification method. By this method, it becomes possible to grasp the accurate content of Polygalaxanthone III, which will becomes an important and effective means for conducting on-site quality assessments in the future.