Abstract: An experiment was conducted in the poultry farm of Al-Shouback University College in the campus of Al-Balqa` Applied University to evaluate the effect of using a medicinal plant seed powder of black cumin (Nigella sativa L.) as a natural feed additive on the performance of 4-8 week-old broilers. Two hundred and forty growing broilers at 4 weeks of age were weighed and randomly distributed into four treatments, with 3 replicates. The seed powder of N. sativa was daily prepared and mixed with the broiler’s finisher diets at rates of 1.0, 1.5 and 2.5% per km of broiler’s finisher diet. Control group received a complete finisher diet free of N. sativa seed powder and any additive. The experimental diets of N. sativa seed powder were offered Ad libitum during the age 4-8 weeks of lohman broilers reared in an open-sided poultry house. The results showed that, feeding of the diets with 1 and 1.5% N. sativa seeds for a period of 4 weeks was significantly (p<0.05) increased the body gain of broilers by 10 and 14.5%, respectively. Broilers fed 1 and 1.5% of N. sativa had significantly (p<0.05) lower feed to gain ratio than other treatments, the best feed efficiency was achieved by broilers given 1.5% N. sativa in their diet. Increasing the feeding rate of N. sativa up to 2% was associated with a significant (p<0.05) decrease of -2 and -8% in body gain and body weight, respectively. Inclusion of N. sativa seeds in the diet reduced the cumulative rate of mortality. Based upon this study, it could be concluded that using the medicinal seed powder of black cumin ( Nigella sativa L.) as a natural feed additive at a rate of 1.5% revealed positive effects on the performance and survivability of broilers grown at normal conditions in an open sided environment.