In the course of our search for antitumour constituents from the traditional Mongolian medicinal herb Clematis aethusifolia Turcz., 11 flavonoids were isolated for the first time from the dried aerial parts of the plant by flash C18 column chromatography, Sephadex LH-20 and reversed phase preparative HPLC. The planar structures of these flavonoids were established based on 1D and 2D NMR and high-resolution mass spectrometry. Compounds 1, 2, 4 and 5 showed moderate cytotoxicity against a panel of five human solid tumour cell lines, including A-375, a human melanoma cell line; SK-OV-3, a human ovarian cancer cell line; A549, a human lung cancer cell line; HCT-15, a human colorectal adenocarcinoma cell line; and SH-SY5Y, a human neuroblastoma cell line (with IC50 values of 20-70 μM). The obtained cytotoxic apigenin and its derivatives may be useful as standard compounds for the quality control of the crude drug and its preparations.