The structure of thalictine (1), isolated from the stems and leaves of Thalictrum thunbergii DC., has been established as 1 by nuclear magnetic resonance (NMR) spectral data and chemical degradation. The molecular formula C₃₇H₄₀O₆N₂ of thalictine was determined by elemental analysis and mass spectrometry, indicating it is a bisbenzylisoquinoline (biscoclaurine) alkaloid. Chemical degradation methods including permanganate oxidation of O-ethylthalictine methyl methine (yielding 4-ethoxy-3,4'-oxydibenzoic acid, identified with an authentic sample) and sodium in liquid ammonia cleavage of O-ethylthalictine (yielding (S)-armepavine and (S)-O-ethylarmepavine) were employed. NMR spectral data revealed that thalictine is a 5–7' linked biscoclaurine alkaloid with a 21-membered heterocyclic ring, and its stereostructure was established as (S,S)-type. Mass spectral data and molecular models further supported the structure assignment. Thalictine is an additional example of 5-7' linked biscoclaurines and contains a cryptophenolic hydroxyl group in the benzyl moiety.