A new flavanol derivative, (2R,3R)-3-acetoxy-7-hydroxy-3',4'-methylenedioxyflavan (1), was co-isolated from the rhizomes of Zephyranthes ajax Hort. with the following seven known compounds: 7-hydroxyflavan (2), 7,4'-dihydroxyflavan (3), 7,4'-dihydroxy-8-methylflavan (4), 7,3'-dihydroxy-4'-methoxyflavan (5), 5,4'-dihydroxy-7-methoxy-6-methylflavan (6), 7-hydroxy-3',4'-methylenedioxyflavanone (7) and haemanthamine (8). Their structures were elucidated by combining 1D-/2D-NMR, CD, UV and HRESIMS data, and comparisons with reported data in literature were made. Among these known compounds, 2, 3, 4, 6 and 7 were isolated from the genus Zephyranthes for the first time. In addition, the cytotoxicity assay indicated that compound 8 has potent cytotoxic activity against human hepatocellular carcinoma (the HepG2 cell line), human lung carcinoma (the SK-LU-1 cell line), human carcinoma in the mouth (the KB cell line), human colon carcinoma (the SW480 cell line) and human stomach gastric adenocarcinoma (the AGS cell line), with IC50 values ranging from 4.4 to 11.3 µM. This is the first study reporting the cytotoxicity of compound 8 against the SK-LU-1 cancer cell lines.