Abstract:Objective: To investigate the chemical constituents of the red alga Gymnogongrus flabelliformis Harv. Method: Compounds were isolated by normal phase silica gel and Sephadex LH-20 gel column chromatography and reverse phase HPL C. Their structures were elucidated by spectroscopic methods including MS, ¹H-NMR、¹³C-NMR. Cytotoxicity of the compounds was screened by using standard MTT method. Result: Five compounds were isolated from G. flabelliformis, their structures were identified as(3S, 6R, 7E)-(+)-3-hydroxyl-4, 7-mega-stigmadien-9-one (1), (3S, 5R, 6S, 7E)-(-)-3-hydroxy-5, 6-epoxy-7-megastigmene-9-one (2), (3S, 5S, 6R, 7E)-(+)3-hydroxy-5, 6-epoxy-7-megastigmene-9-one (3), dehydrovomifoliol (4), (3R)-(-)-4-［(2R, 4S)-4-acetoxy-2-hydroxy-2, 6, 6-trimethylcyclohexylidene］-3-buten-2-one (5)。Conclusion: All of the compounds were obtained from this species for the first time and compound 1 was a new natural product. These compounds were inactive (IC₅₀>10 μg·mL^-1) in the MTT assay against several human cancer cell lines.