Two new C18-diterpenoid alkaloids, sinomontadine (1) and sinomontanine N (2), were isolated from Aconitum sinomontanum. Their structures were determined by spectroscopic methods and X-ray crystallographic analysis. Sinomontadine (1) features an unprecedented skeleton containing a seven-membered ring (A-ring), while sinomontanine N (2) is chlorinated. A plausible biogenetic pathway for 1 was proposed. © 2017 Elsevier Ltd