Effects of various concentrations of garlic powder and garlic extract in the diets on growth, serum chemistry and immune response of olive flounder were determined. Thirty-five juvenile fish averaging 5.1 g were randomly distributed into 21 of 180 L flow-through tanks. Seven experimental diets with various concentrations of garlic powder (GP) and garlic extract (GE) were prepared in triplicate: GP-0 without garlic supplementation, GP-0.5, GP-1, GP-2, GP-3 and GP-5 diets containing garlic powder at the concentrations of 0.5, 1, 2, 3 and 5%, respectively at the expense of wheat flour and finally, GE-0.4 diet containing 0.4% garlic extract were prepared. At the end of the 8-week feeding trial, serum chemistry of fish was measured. In addition, twenty fish from each tank were artificially infected with E. tarda for the following 96 h to monitor cumulative mortality. Weight gain of fish fed GP-0 diet was higher than that of fish fed GP-1, GP-2, GP-3 and GP-5 diets. No difference in serum criteria (total protein, glucose, glutamate oxaloacetate transaminase, cholesterol and triglyceride levels) of olive flounder was found among the experimental diets except for glutamate pyruvate transaminase. Lysozyme activity of fish fed GP-0, GP-1, GP-3 and GE-0.4 diets was higher than that of fish fed GP-5 diet. The highest cumulative mortality was 93.3% in fish fed GP-0 diet at 96 h after E. tarda infection, followed by GP-3, GP-1, GP-5, GP-2, GP-0.5 and GE-0.4 diets. In considering these results, dietary inclusion of garlic powder and garlic extract has no distinctive positive effect on improvement in growth, serum chemistry and immune response of olive flounder in this experimental conditions, therefore, its application should be carefully considered.