The ethanol extract of Chrysanthemum sinense Sabine, Asteraceae, flowers was found to inhibit xanthine oxidase with a IC50 value of 8.20 ± 1.36 µg/ml. Using various chromatographic methods, one new guaianolide-type sesquiterpenoid, chrysanoside K, along with seventeen known compounds, cumambrin B, angeloylcumabrin B, 3α-chloro-4β,10α-dihydroxy-1β,2β-epoxy-5α,7αH-guai-11(13)-en-12,6α-olide, zawadskinolide B, jaceidin, quercetagetin 3,6,3ʹ,4ʹ-tetramethyl ether, tilianin, apigenin 7-O-β-D-glucopyranoside, apigenin 7-rutinoside, linarin, eriodictyol 7-O-β-D-glucopyranoside, naringenin 7-O-β-D-glucopyranoside, (9E)-4-oxo-7-hydroxy-11-hydroperoxy-bisabola-2,9-diene, indicumenone, indicumenone 11-methyl ether, 7-hydroxy-3,10-bisaboladien-2-one, and 5-acetoxy-9-oxonerodidol were isolated from the flowers of C. sinense. Their structures were elucidated by spectroscopic methods and compared with those reported in the literature. All isolated compounds were evaluated for their inhibitory effect on xanthine oxidase activity. As results, compounds jaceidin, quercetagetin 3,6,3ʹ,4ʹ-tetramethyl ether, and apigenin 7-O-β-D-glucopyranoside significantly inhibited xanthine oxidase activity with IC50 values of 1.08 ± 0.16, 8.59 ± 1.09, and 5.93 ± 0.45 µM, respectively.