Our objective was to investigate the effects of NS, Nigella Sativa oil (NSO) and thymoquinone (TQ) on liver fibrosis induced by dimethylnitrosamine (DMN) in rats. Our study groups consisted of five subgroups: the control, DMN, TQ, NS, and NSO groups. At the end of the study, the animals were sacrificed, and blood and tissue samples were obtained. Malondialdehyde (MDA) levels, superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) activity, and routine biochemical parameters were analyzed. Histopathologic evaluation of the NS, NSO and TQ groups found normal liver tissue and short fibrosis, and fibrous expansion with or without a septum and in some portal areas. The DMN group evaluation found incomplete cirrhosis in the liver tissues of rats. While erythrocyte and tissue MDA levels were higher, SOD and GSH-Px activities were lower in the NS, NSO and TQ groups when compared to the DMN group. Our results showed that NS and its components reduced oxidative stress and had antioxidant effects and free-radical scavenging activity. NS also protected the antioxidant enzyme activities in the liver tissue of rats.