BACKGROUND: Graptopetalum paraguayense E. Walther is a popular traditional Chinese herb and possesses several health benefits. In earlier studies, we demonstrated that G. paraguayense showed no genotoxicity and showed several biological activities. However, the constituents of G. paraguayense have not been studied yet. In this present study, we isolated and identified the constituents of the leaves of G. paraguayense E. Walther. RESULTS: A total of seven flavonoid compounds were isolated from the methanolic extract of G. paraguayense. The four major compounds isolated were flavonoid glucoside derivatives of quercetin (1, 3) and kampferol (2, 4), each presenting a 3-hydroxyl-3-methylglutaroyl (HMG) substituent; compounds 3 and 4-the 2´´-acetyl derivatives of 1 and 2, respectively-are novel compounds isolated from nature for the first time. High-performance liquid chromatography for the quantitative analyses of the four major HMG-substituted flavonoid glycosides in G. paraguayense E. Walther were accomplished to acquire the high yields of 1-4 in the methanolic extract (4.8, 5.7, 4.3, and 2.5 mg/g, respectively). Furthermore, the antioxidant activities, including radical-scavenging, reducing power and lipid peroxidation inhibitory effects of these isolated flavonoids were also evaluated. All seven of the isolated flavonoid compounds possessed antioxdative activity. CONCLUSIONS: In this study of the constituents of the leaves of G. paraguayense E. Walther, we isolated four major components from its methanolic extract and determined their structures to be (acetylated) HMG-substituted flavonol glycosides, which are rare in nature. All seven of the isolated compounds possessed antioxdative activity, and those flavonoid compounds may be responsible for the functional ingredients in G. paraguayense. Further investigation of their bioactivities or pharmacological activities will be continued.