Oxidative stress has been known as the main cause of the development of many diseases. Therefore, supplementation of exogenous antioxidants and elevating of endogenous antioxidant defenses may be the effective way of combating the harmful action of reactive oxygen species, which considered the main reason of oxidative cell damage. The study was conducted to identify the main polyphenolic components of Brassica juncea seeds and investigate their cytotoxic effects against different cancer cell lines. Also, 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS^{. +}) antioxidant assay and erythrocyte hemolysis were tested against ascorbic acid. The cytotoxic activity of B. juncea on all cancer cell lines was significantly higher than the chemotherapeutic drug doxorubicin (Dox). These results confirm that B. juncea has a strong cytotoxic effect on most tumor cells; the viability of the six cancer cell lines was significantly decreased when using B. juncea extract compared to doxorubicin. B. juncea extract showed cytotoxic effect on cancer cells compared to Dox, however, appropriate dose must be determined