Rhabdasterol - 25 (29)-methylene-26-methyl-26-ethycholeste. a novel marine sterol with unusual side chain has been isolated from a yellow sponge Rhabdastrella sp., which was collected from South China Sea. The chemical structure has been determined by spectral data of IR, HRMS, EIMS, H-1 NMR, C-13 NMR and TOCSY etc..