The use of dietary additives in fish farms is one of the methods commonly used to improve weight gain, feed efficiency, and/or disease resistance in cultured fish. Allium sativum (garlic) and A. cepa (onion) are herbs or herbal derivatives that can replace antibiotics and other harmful growth promoters. The present study deals with the influence of different levels (0.5 or 1.0%) of each of  A. sativum and A. cepa extract in fish diet as a natural growth promoter on growth performance, nutrient utilization and hematological parameters  of mono-sex all male Nile Tilapia (Oreochromis niloticus) fingerlings. Five treatments were triplicated and a total number of 1500 fish (14.0±0.19 g) were stocked in 15 cages (2×4×1 m) in earthen pond. At the end of the experiment (after 120 days), the highest (P<0.05) weight gain (WG) and specific growth rate (SGR) were observed in fish groups fed the diets supplemented with 1% garlic or onion extracts. Also, the different diets showed no significant different on feed conversion ratio (FCR). After 120 days, the fish groups fed the diets supplemented with garlic and onion extracts showed a significantly (P<0.05) increase in the number of erythrocytes (RBC), leucocyte (WBC) compared to the control group. Albumin (g/dl), total proteins g/dl and haemoglobin (g/dl) levels had significant change (P>0.05) in treatment groups and control. The results of this study demonstrated that dietary1.0% garlic or onion extracts could improve growth, hematological parameters and immune function of mono-sex all male Nile Tilapia fingerlings reared in cages in earthen pond system .