As part of an ongoing search for natural plants with antioxidant compounds by measuring the radical-scavenging effect on DPPH (1,1-diphenyl-2-picrylbydrazyl), a total extract of the whole plant of Actinostemma lobatum (Cucurbitaceae) was found to show potent antioxidant activity. Subsequent activity-guided fractionation of the methanolic extract led to the isolation of eight compounds: actinostemmoside C (1), kaempferol-3-O-${alpha}$-L-rhamnopyranosyl ($1{ ightarrow}6$)-${eta}$-D-glucopyranoside (2), quercetin-3-O-${eta}$-L-rhamnopyranosyl ($1{ ightarrow}6$)-${eta}$-D-glucopyranoside (3), quercetin-3-O-${eta}$-D-glucopyranoside (4), kaempferol (5), quercetin (6), lobatoside C (7), and lobatoside B (8). Their structures were elucidated using spectroscopic studies. Compounds 2, 4, 5, and 6 were isolated for the first time from this plant. Compounds 2-4 and 6 showed significant antioxidative effects during the DPPH-free radical-scavenging test and the superoxide quenching activity test.