The aim of this study was to investigate the antioxidant properties of the root extract of Onosma armeniacum and its cytotoxic effect against human liver (HepG2), lung (A549) and colon (WiDr) cancer cell lines compared with a normal fibroblast cell line. Antioxidant properties of the extract was determined using the colorimetric methods. The cytotoxic effect of the extract were investigated using MTT assay. The total phenolic content and reducing power values of extract were 26.1 +/- 0.4 mg gallic acid equivalent and 52.6 +/- 1.1 mg trolox equivalent per g sample, respectively. The extract exhibited selective cytotoxic effect in all studied cancer cell lines, especially HepG2 (3.6-fold). The results of this study reveal that O. armeniacum, which has been found to have acceptable antioxidant and cytotoxic effects, should be examined with more comprehensive studies.