Two new C(29) sterols with a cyclopropane ring at C-25 and C-26, petrosterol-3,6-dione (1) and 5alpha,6alpha-epoxy-petrosterol (2), along with petrosterol (3), were isolated from the Vietnamese marine sponge Ianthella sp. The structures of the new compounds were elucidated by comprehensive spectroscopic analyses. Compounds 1-3 showed cytotoxic activities on A549, HL-60, MCF-7, SK-OV-3, and U937 cancer cell lines with IC(50) in the range of 8.4-22.6 microM, whereas compounds 1-3 exhibited only weak cytotoxic activities on HT-29 cell. After HL-60 cells were treated with the compounds, several apoptosis events like chromatin condensation and the increase of the population of sub-G1 hypodiploid cells were observed. These data supported that the compounds might have potential for leukemia treatment.