Antioxidant potencies of an ethanolic extract and its components from Lepidium latifolium were investigated. In this study, we found that the ethyl acetate soluble fraction of L. latifolium was a rich source of antioxidant, resulting from its high total phenolic content. To determine the antioxidant components of the ethyl acetate fraction, a bioassay-guided fractionation approach using 1,1-diphenyl-2-picrylhydrazyl, 2,2′-azinobis-(3-ethylbenzthiazoline-6-sulfonate), and ferric reducing antioxidant power assays were conducted. Nine compounds were isolated and their structures were identified by MS and NMR spectral data and comparison to reported data. They are Quercetin-3-O-β-d-sophoroside-7-O-α-l-rhamnoside (1), Apetalumoside B6 (2), Kaempferol-3-O-β-d-glucopyranosyl-(1-2)-β-d-glucopyranoside-7-O-β-d-glucopyranoside (3), Kaempferol-7-O-α-l-rhamnopyranoside (4), Kaempferol-3-O-β-d-glucopyranoside-7-O-α-l-rhamnopyranoside (5), Kaempferol-3-O-(2-O-feruloyl-β-d-glucopyranosyl-(1-2)-β-d-glucopyranoside)-7-O-glucopyranoside (6), Kaempferol-3-O-β-d-sophoroside-7-O-α-l-rhamnoside (7), Kaempferol-3-O-robinoside-7-O-(2″″-(E)-feruloyl)-sophoroside (8), Quercetin-3-O-(2,6-di-O-β-d-glucopyranosyl)-β-d-glucopyranoside-7-O-α-l-rhamnopyranoside (9), compounds 1, 2, 4, and 8 had potent free radical scavenging activity. The IC₅₀ values of these compounds were 9.8-12.3 and 7.4-31.4 μg/mL in DPPH and ABTS assays, respectively. The results indicate that L. latifolium is a potential natural source of antioxidants to treat several diseases related to oxidant by-products of human metabolism.