The objective of this experiment to study the immunomodulation response of garlic as a feed additive for Nile tilapia. Therefore, to investigate the influence of this additive on the immune hematological parameters. Non-specific immune response of Nile tilapia and diseases resistance abilities will be evaluated. The current study demonstrated that garlic, Allium Sativum can be used to modulate the immune system of Nile tilapia to the favorer of resistance to diseases. Two diet regimes basic, (control), and 3% garlic were formulated and used to feed fish for 60 successive days. Fifty fish were used to investigate some of the immune parameters as serum globulins, total and differential white blood cell counts, phagocytic activities and phagocytic indices. The other fifty fish was subjected to infection challenge with Aeromonashydrophila to investigate the disease resistance ability of the fish received the feed additive. Results showed that the white blood cell counts, serum globulins, phagocytic activities and phagocytic indices have significantly increased in fish fed on 3% garlic supplemented diet than the control one. The mortality rate of fish fed on 3% garlic supplemented diet and challenged with Aeromonashydrophila were significantly lower than those that received basal diet. Obtained results concluded that garlic has an immunostimulative effect and garlic powder could be recommended to be used for farmed fish to decrease mortalities caused by pathogenic microorganisms.