This study aimed to investigate the impact of supplementing different levels of Nigella sativa oil in rations of growing lambs. In a feeding experiment lasted 90 days, 18 growing Ossimi lambs (6-7 months old) were randomly divided by weight into three equal groups (6 animals each). Individual animals in all groups were fed a uniform total mixed ration, offered at 3% of body weight, where the first group was without supplement (R1), while Nigella sativa oil was supplemented at 0.1 (R2) and 0.2% (R3) of dietary DM. Results indicated that additions of N. sativa oil didn’t influenced feed intake, but it increased average body weight gain and feed efficiency. Nutrients digestibility values and dietary nitrogen utilization were higher with N. sativaoil rations than control. Ruminal fermentation parameters showed comparable pH values among groups at 0, 3 and 6 hrs. of feeding. Ruminal NH3-N concentration was lower and TVFA concentration was higher for animal fed R2 and R3 rations than control. It’s concluded that N. sativa oil could be added at 0.2 % of the daily feed as a natural growth enhancer of lambs.