Three new C19 -norditerpenoid alkaloids (1-3), along with two known C19 -norditerpenoid alkaloids (4-5) have been isolated from Aconitum szechenyianum. Their structures were established by extensive spectroscopic techniques and chemical methods as szechenyianine A (1), szechenyianine B (2), szechenyianine C (3), N-deethyl-3-acetylaconitine (4), and N-deethyldeoxyaconitine (5). Additionally, compounds 1-5 were tested for the inhibition of NO production on LPS-activated RAW264.7 cells with IC50 values of 36.62 ± 6.86, 3.30 ± 0.11, 7.46 ± 0.89, 8.09 ± 1.31, and 11.73 ± 1.94 μM, respectively, while the positive control drug dexamethasone showed inhibitory activity with IC50 value of 8.32 ± 1.45 μM. The structure-activity relationship of aconitine alkaloids were discussed. © 2016 by the authors; licensee MDPI, Basel, Switzerland.