A new C₂₁ steroid (compound A) has been isolated from Marsdanid tendcissszma (Roxb.) Wight et Arn.(Aedepiadaoeae).The molecular formula of compound A is ₂₁H₃₂O₅ Compound A does not show carbonyl absorption and olefinio proton resonance in its IR and NMR spectra which is quite different from commonly oooured Ca₂₁ steroids.The five oxygen atoms in compound A have been proved to be present as three hydroxyl group and two ozygen containing rings.By means of chemical and spectroscopic analysis as well as X-ray diffraction method,the ahemioal structure of compound A has been determined as 5α,17α-pregnane-3β,8α,11α,12β,14β-pentaol-20-one-8,11-ketal(3),in which ring B and ring C are cis fused.Moreover,ring C and ring D together with the ketal rings form a rigid cage in this compound.Compound A has been named tenaoissigenin.