Guan‐fu base G is a new alkaloid isolated from root tuber of Aconitum coreanum Le'vl Raipaics. Guan‐fu base G methyliodide crystallizes in orthorhombic system, space group D4/2‐P212121, with dimensions: a = 16.87Å, b = 16.93Å, c = 10.09Å; Z = 4. The three dimensional data were collected on a Phillips PW‐1100 four‐circle diffractometer with Mo Ka radiation. The structure was solved by heavy‐atom method and subsequently successive Fourier syntheses. The structure was refined by block‐diagonal matrix least squares methods to R = 0.0578 for 2848 reflexions. The absolute configuration was determined by computation of the R factor for the two possible configurations using anomalous scattering of iodine. Copyright © 1984 SIOC, CAS, Shanghai & WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim