By various chromatographic separations, four known compounds, 2,2-bis[4-(2,3-dihydroxypropoxy)phenyl]propane (1), 2,2-bis[4-(2,3-epoxypropoxy)phenyl]propane (2), cholest-7-en-3beta,5alpha.,6beta-triol (3) and 29-hydroxystigmasta-5,24(28)-dien-3beta-ol (4) were isolated from the sponge Stylissa flabelliformis. Their structures were elucidated by spectroscopic methods and in comparison with the reported data. All isolated compounds were evaluated for cytotoxic activity on human cancer cell lines, including HepG-2, KB, LU-1, MCF-7 LNCaP, SW-480, MKN-7, and HL-60. As the results, compounds 3 and 4 exhibited moderate activities in all human cancer cell lines with the IC50 values of 10.32 - 26.06 ug/mL.