Five new 7-methoxy-flavone 5-O-glycosides were isolated from a cytotoxic MeOH extract of Lethedon tannaensis, and the structures were elucidated by 2D NMR spectral analysis and by chemical methods. Lethedosides A (1), B (2), and C (3) were 5-O-glucosides of 7,3', 4'-tri-O-methylluteolin, 7,3',4',5'-tetra-O-methyltricetin, and 7,3', 4'-tri-O-methytricetin, respectively; lethediosides A (4) and B (5) and a known compound 6 were 5-O-xylosylglucosides of 7,3', 4'-tri-O-methylluteolin, 7,3',4',5'-tetra-O-methyltricetin, and 7, 4'-di-O-methylapigenin, respectively. These flavonoids were either inactive or weakly active against KB tumor cells, in contrast to previously isolated flavones from the same plant.