Twenty three diterpenoid alkaloids (1-23) have been isolated from Aconitum japonicum Thunb. and their structures were ditermined on the basis of chemical and spectral evidences. Secojesaconitine (19) has an unusual skeleton, and the structure was determined by X-ray analysis. By treatment with acetic acid and acetylation, 19 gave normal aconitine type alkaloid. Acetyloxy group at C_8 position in the aconitine type alkaloid was readily exchanged for alkoxy group in alcoholic media (16 and 17). Aconitine (4) was transformed into penduline (8) by three steps involving deoxygenation of bridgehead hydroxyl. Analgesic activity of aconitine and its related compounds was investigated by acetic acid-induced writhing method. The acute toxicity of 8-O-methyl-14-benzoylaconine was much lower and the activity was approximately same as morphine's.