One new $C_{29}$ sterol with a cyclopropane ring at C-25 and C-26, aragusteroketal B (1), and aragusterol B (2) were isolated from the Vietnamese marine sponge Ianthella sp. Their structures were elucidated by extensive spectroscopic analyses. Both 1 and 2 showed moderate cytotoxic activity against MCF-7, SK-Hep-1, and HeLa cell lines with $IC_{50}$ in the range of $12.8-27.8;{mu}M$.