A large part of the primary malignant tumors of the liver is hepatocellular carcinoma. Diethylnitrosamine (DEN) constitutes a large part of genetically effective chemical carcinogens. It is known that stinging nettle, which is widely used in alternative medicine today, has many medicinal properties. In the present study, the antitumor activity of Urtica dioica was investigated in the process of hepatocellular carcinogenesis induced by diethylnitrosamine in rats. UDS group rats were given 1 ml of UDS extract (0.5 ml ether oil extract and 0.5 ml ethanol extract) orogastrically daily for 16 weeks. DEN group were given 200 mg/kg DEN once intraperitoneally. DEN+UDSE-1 group were given 200 mg/kg DEN once intraperitoneally and 1 ml of UDS ethanol extract orogastrically daily for 16 weeks. DEN+UDSE-2 group were given 200 mg/kg DEN once intraperitoneally and 1 ml of UDS ether oil extract orogastrically daily for 16 weeks. Biochemically; antitumor activity was evaluated by investigating TOS, MDA, SOD, GSH-Px, CAT, TAS, AST, ALT, LDH, CA 15-3, CA 19-9 and CA 125-II activity/levels. Pathologically, in the DEN group, there was a fine granular appearance on the surface of the livers. Histopathologically, the dysplasias in the form of large and small cell changes were commonly observed in hepatocytes. Additionally, bile duct hyperplasia, inflammatory cell infiltrations, fibrosis and cholestasis were seen in the liver parenchyma. In the groups receiving UDSE with DEN, dysplastic large cell changes, cystic degenative hepatocytes and fibrosis were not observed, however the other pathological changes were at mild levels. According to these results, in the process of hepatocellular carcinogenesis induced by diethylnitrosamine, it was concluded that UDS extract has a very significant antitumor activity by preventing tissue damage due to its strong antioxidant capacity.