Abstract: Aquatic pollutants are responsible form any alterations in fish health. Our study was designed to examine the efficacy of dietary supplement of black mustard seed, Brassica nigra (30% crude and 1% alcoholic extract) on enhancement the immunity and detoxification response of Nile tilapia, Oreochromis niloticus after exposed to a hydrocarbon pollutant; Benzo-a-Pyrene (BaP). Fish were divided into 3 groups before being fed for 28 days with crude black mustard seeds,1% alcoholic extract and un-supplemented commercial diet as the control. At the end of experiment, fish were exposed to 1 mg L–1 of BaP for 24 h. Humoral immune parameters (lysozyme activity, antiprotease activity and total protein),ethoxyresorufin-o-deethylase (EROD) and hepatic cytochrome P450 1A1 content (CYP1A1) were investigated. Results recorded high significant difference (p<0.05) of antiprotease and total protein values in group fed with 1% extract followed by crude seeds as compared with control. The highest significant lysozyme activity (p<0.05) was recorded also in group fed with 1% extract as compared with other groups. On the other hand the extract group revealed a significant decrease in EROD (p<0.05) and CYP1A1 content compared to other groups. Which lowering the possible toxic effect of bio-transformed metabolic intermediate compounds. While the EROD activity and CYP1A1 content in group fed with the crude seeds showed increase over the control one. Therefore, the results suggest using of black mustard extract as a dietary supplement to increase the immune function and enhance the detoxification process of pollutants and hence, reduce the harmful effect of aquatic pollutants on Nile tilapia health.