In the course of screening for antioxidant compounds by measuring the DPPH (1,1-diphenyl-2-picrylhydrazyl) radical scavenging effect and superoxide quenching activity, methanol extract of Cyperus flaccidus (Cyperaceae) was found to show potent antioxidant activities. Subsequent activity-guided fractionation of the methanolic extract led to the isolation of ten compounds, kaempferol-3,7-O-${alpha}$-L-dirhamnopyranoside (1), caffeic acid (2), quercetin-3-O-${alpha}$-L-rhamnopyranosyl($1{ ightarrow}6$)-${eta}$-D-glucopyranoside (3), kaempferol-3-O-${alpha}$-L-rhamnopyranoside (4), quercetin-3-O-${alpha}$-L-rhamnopyranoside (5), luteoiln-7-O-${eta}$-D-glucopyranoside (6), kaempferol-3-O-${alpha}$-L-rhamnopyranosyl($1{ ightarrow}6$)-${eta}$-D-glucopyranoside (7), luteoiln (8), quercetin (9) and quercetin-3-O-${eta}$-D-glucuronide (10). Their structures were elucidated by spectroscopic studies. Compounds 1 - 10 were isolated for the first time from this plant. Among them, compounds 2, 8 and 9 showed the significant radical scavenging effects on DPPH, and the potent xanthine-originated superoxide quenching activities.