Flavonoids contents in 40 samples of Semen Cuscutae collected from areas all around China were investigated. Five principal flavonoids, quercetin 3-O-beta-D-galactoside-7-O-beta-D-glucoside, quercetin 3-O-beta-D-apiofuranosyl-(1-->2)-beta-D-galactoside, hyperoside, quercetin and kaempferol were analyzed simultaneously by using a reversed phase liquid chromatograph system with 0.025 M phosphoric acid-methanol as mobile phase. The recovery of the method was 97.0-102.9%, and all the flavonoids showed good linearity (r > or = 0.9990) in a relatively wide concentration range. The results indicated that contents of flavonoids in S. Cuscutae varied significantly from species to species, locality to locality, and parasiting host to host. Cuscuta australis contained a much higher content of kaempferol than C. chinensis, and few flavonoids were detected in C. japonica. The quality of S. Cuscutae can be evaluated according to the contents of flavonoids.