Abstract: Tenacigenin A was obtained from crude glucoside of Marsdenia tenacissima (Roxb.) Wight et Arn., a folk antitumor drug in Yunnan, by treatment with acid and base. This compound C21H32O5, mp 255–260℃, belonging to C21-steroidal type, readily yielded a diacetate (Ⅲ) and under forcing condition a triacetate (Ⅳ). Two of the three hydroxyl groups were able to be oxidized to diketone (Ⅵ), which were attached to the six-membered rings. The other two oxygenatoms existed as in the form of epoxy. Tenacigenin A afforded 3β, 12β, 14β-triacetoxy-5α-pregn-7(8), 9(11)-diene- 20-one (Ⅸ), which was hydrogenized with Pt in AcOH to (Ⅹ), by successive treatment with Ac2O and PTS at 100 ℃ for 30hr. Accoring to the above chemical reactions (Fig. 3) and data of IR, UV, NMR (Fig. 1) and MS (Fig. 2), the structure of tenacigenin A has been suggested as (Ⅰ).