Two consecutive studies were conducted to evaluate the effects of dietary supplementation with fermented garlic powder (FGP) or fermented garlic fluid (FGF) on growth performance, immune responses, and disease resistance of olive flounder Paralichthys olivaceus. In experiment I, olive flounder (BW: 65 g) were fed four experimental diets formulated to contain 0%, 0.5%, 1%, and 1.5% FGP (designated as FGP-0, FGP-0.5, FGP-1, and FGP-1.5, respectively). After the 10-weeks feeding trial, feed intake was significantly lower in fish fed the FGP-0.5 and FGP-1.0 diets, as compared to those fed the control diet. Fish fed the FGP-0 and FGP-0.5 diets showed significantly lower survival, as compared to the other treatments. Dietary supplementation with FGP resulted in higher non-specific immune responses than the FGP-0 group. Plasma cholesterol and triglyceride levels decreased as dietary FGP level increased. In experiment II, olive flounder (BW: 65 g) were fed four experimental diets for 10 weeks. The diets were prepared with a commercial expanded pellet to have 0%, 0.25%, 0.5%, and 1% FGF (designated as FGF-0, FGF-0.25, FGF-0.5, and FGF-1, respectively) by adsorption. At the end of the second feeding trial, feed intake was significantly lower in fish fed the FGF-0 diet, as compared to other treatments. Fish fed the FGF-0.25 and FGF-0.5 diets exhibited significantly lower cholesterol levels, as compared to other treatments. Lysozyme activity significantly increased with increases in dietary FGF. Cumulative mortality in a challenge test with Streptococcus iniae was significantly lower in the fish groups fed FGF-supplemented diets than in fish fed the control diet. The results of this study indicated that dietary supplementation with FGP or FGF can enhance the non-specific immune responses and disease resistance of olive flounder against S. iniae.