As a component of our continuing investigations into herb-derived antioxidant agents, we have evaluated the antioxidant effects of Flos Lonicerae (Lonicera japonica flowers), via 1, 1-diphenyl-2-picrylhydrazyl (DPPH) radical, total reactive oxygen species (ROS), hydroxyl radical (OH), and peroxynitrite $(ONOO^-)$ assays. Among the methanolic extract and the dichloromethane, ethyl acetate, n-butanol, and water fractions, the EtOAc fraction of Flos Lonicerae exhibited marked scavenging/inhibitory activities, as follows: $IC_{50}$ values of 4.37, $27.58{pm}0.71,;0.47{pm}0.05,;and;12.13{pm}0.79{mu}g/mL$ in the DPPH, total ROS, $ONOO^-$, and OH assays, respectively. Via a bioactivity-guided fractionation approach, a new triterpenoid glycoside, oleanolic acid $28-O-{alpha}-L-rhamnopyranosyl-(1{ ightarrow}2)-[{eta}-D-xylopyranosyl(1{ ightarrow}6)]-{eta}-D-glucopyranosylester$ (12), along with eleven known compounds, including chrysoeriol(1), luteolin (2), 5-hydroxymethyl-2-furfural (3), caffeic acid (4), protocatechuic acid (5), chrysoeriol $7-O-{eta}-D-glucopyranoside$ (6), isorhamnetin $3-O-{eta}-D-glucopyranoside$ (7), kaempferol $3-O-{eta}-D-glucopyranoside$ (8), quercetin $3-O-{eta}-D-glucopyranoside$ (9), hederagenin $3-O-{alpha}-L-arabinopyranoside$ (10), and luteolin $7-O-{eta}-D-glucopyranoside$ (11), were isolated from the EtOAc fraction. The structures of isolated compounds 1-12 were elucidated via spectroscopic analyses. Compound 12 was isolated from a natural source for the first time. Compounds 2, 4, 5, 7, 9, and 11 evidenced marked scavenging activities, with $IC_{50}$ values of $2.08-11.76{mu}M$ for DPPH radicals, and $1.47-6.98{mu}M;for;ONOO^-$.