Objective To investigate the effect and mechanism of radix Sophorae flavescentis on the trophozoites of Giardia lamblia in vitro. Methods Trophozoites of G. lamblia were exposed to aqueous extracts of radix S. flavescentis and then the detachment from the tube wall, the death rate and the morphological changes under light and transmission electron microscope of trophozoites were studied. Results After exposure to 1.6% aqueous extracts of radix S, flarescentis for 6, 8 and 24 h, the detachment rates were 47. 4%, 70. 9% and 80. 2%, and the death rates were 24. 5%,36.1% and 54.2%, respectively. The radix S, flavescentis-treated trophozoites shrinked and became roundish. The rewere also changes in ultrastructure, such as depletion of the contents of cytoplasm, separation of adhesive disk from the cell body and appearance of pit in cell membrane of some treated trophozoites, Conclusion Radix S, flavescentis can kill trophozoites of G, lamblia and lead them to detach from the tube wall in vitro. The induced morphological changes may be partially responsible for the effects of radix S, flavescentis on giardiasis.