To examine the effect of supplementation of varying levels of the oil of Nigella sativa in Karadi lambs rations on nutrients digestibility, blood metabolites, and some rumen parameters, 18 Karadi lambs were allocated into three groups, and the first group was fed a basal diet as control whereas, the second (T2) and the third (T3) groups fed the basal diet being supplemented with 0.15 and 0.30% of DM Nigella sativa oil (NSO) respectively. All animals were fed individually on 1.5 kg/lamb/day. Results showed that dry matter (DM), crude protein (CP), organic matter (OM), and crude fiber (CF) digestibility was not affected (P>0.05) by NSO supplementation. Also, supplementing NSO had no significant effect on serum total protein (TP), albumin (Alb), globulin (Glb), triglycerides (TG), cholesterol (Chol), high density lipoprotein (HDL), and very low-density lipoprotein (VLDL) concentrations. There was an increasing trend (P=0.07) in LDL concentration of lambs fed on T2 and T3 as compared to control. Neither treatment nor interaction between time and treatment had an effect on rumen fluid pH. A significant decrease (P=0.008) was noted in rumen fluid pH value with the advances of time post feeding. The ammonia-nitrogen concentration in rumen fluid was generally lower upon oil supplementation, and it was significantly (P=0.03) decreased in the T2 group at 4 hours following morning feeding. It can be concluded that supplementing with 0.15 and 0.3% /DM of NSO showed a reduction in rumen ammonia-nitrogen while it had no effects on nutrient digestibility and blood metabolites in Karadi lambs.