A new denudatine-type C20-diterpenoid alkaloid bearing a rare C-15 ketone carbonyl and 16,17-epoxy group, named arcutisine (1), together with 24 known compounds (2-25), were obtained from the roots of Aconitum fischeri var. arcuatum. Their structures were established by various spectroscopic analyses. Among them, compounds 1 and 2 were both isolated from a natural source for the first time. Besides, compounds 7 and 10 showed weak effect on anti-inflammatory activity with the inhibition rate of 33.5% and 33.7% at the concentration of 40 μM. © 2021 Japan Institute of Heterocyclic Chemistry. All rights reserved.