The ethyl acetate (EtOAc) layer of the hot water extracts of Camellia japonica flowers was found to have higher 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical-scavenging activity than the other solvent layers. Nine phenolic compounds were isolated and purified from the EtOAc layer by Sephadex LH-20 column chromatography and octadecyl silane-high performance liquid chromatography using a guided DPPH radical-scavenging assay. The isolated compounds were identified as 3,4,5-trihydroxybenzoic acid (1), 3,4-dihydroxybenzoic acid (2), 4-hydroxybenzoic acid (3), 2,3-digalloyl-O-${alpha}$-D-glucopyranoside (4), 2,3-digalloyl-O-${eta}$-D-glucopyranoside (5), quercetin 3-O-${eta}$-D-galactopyranoside (6), quercetin 3-O-${eta}$-D-glucopyranoside (7), kaempferol 3-O-${eta}$-D-galactopyranoside (8), and kaempferol 3-O-${eta}$-D-glucopyranoside (9), based on mass spectrometry and nuclear magnetic resonance. Four compounds (6-9) had been previously identified in the leaves of this plant, but other compounds (1-5) were newly isolated from this plant. Their DPPH radical-scavenging activities based on the 50% scavenging concentration decreased in the following order: 4 = 5 ($4.7{mu}M$) > 1 ($9.8{mu}M$) > 6 = 7 ($8.2{mu}M$) > ${alpha}$-tocopherol ($24.7{mu}M$) > ascorbic acid ($25.1{mu}M$) > 2 ($35.6{mu}M$) > 3 = 8 = 9 ($>250{mu}M$). Quercetin glycosides (6, 7), gallic acid (1) and its glucosides (4, 5) showed higher DPPH radical-scavenging activities than other compounds. These results indicate that the antioxidant effect of C. japonica flowers may be attributable to quercetin glycosides and gallic acid derivatives. These isolated compounds will be useful in basic studies of plant physiology, food manufacturing, and biological function of C. japonica flowers.