A 70% ethanol extract from the leaves of Rosa davurica showed significant antioxidant activity in both superoxide anion and 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical-scavenging assays. Phytochemical study of this extract resulted in the isolation of eight compounds, including a new flavonoid glycoside. The chemical structure was determined by various spectroscopic analyses. The isolated compounds and their structurally related compounds, belonging to two classes: quercetin 3-O-glycosides and gallic acid derivates, were evaluated for their superoxide anion- and DPPH free radical-scavenging activities. These compounds showed significant superoxide anion-scavenging activity with the EC50 values ranging from 1.68 to 18.09 μM, and DPPH free radical-scavenging activity with the EC50 values ranging from 7.18 to 67.62 μM. The structure-activity relationship was also reported.