Abstract: This paper first suggests that 2,4 (1H, 3H) quinazolinedione (QZL) may be used as a quality control standard for the Chinese herbal drugs Daqingye (Folium Strobilanthis et Polygoni) and Banlangen (Radix Strobilanthis) as well as its preparations. QZL was found for the first time from the above mentioned two Chinese herbal drugs which has a representative property. Its content is stable. This made it possible to solve the difficulty of quality control for the preparations "antipyretic and common cold granules" and "Banlangan granules" and provide a reliable assay method to evaluate the quality of the herbal preparations. By changing the wavelength and using gradient elution in HPLC, 3 constituents can be determined through one injection by the present method, thus the time of determination may be shortened and the efficiency of assay increased. The method showed good linearity (r=0. 99998, n=4) in the range of 0.2～3.2 μg/ml of QZL, the average recovery was above. 95% with RSD<1.5% (n=3).HPLC condition: Column: LiChrosorb RP 18; elution gradient: methanol from 35% to 90% in water solution; determination wavelength: 226 nm and 288 nm.