This study was conducted to evaluate the effect of different levels of dietary prebiotic Immunogen on the growth performance, hematological parameters and body composition of koi carp (Cyprinus carpio var. Koi) fingerlings. For this reason, 360 koi carp fingerlings (4.98 ± 0.11 g) were randomly distributed in 12 tanks (n=30 per tank). The fish fed on four different levels of Immunogen (0, 2, 5 and 10 grams of prebiotic per kg diet) for 60 days. The results showed that the highest final weight and specific growth rate (SGR) were observed in the T2 (P<0.05). Minimum condition factor (0.81±0/07) and FCR (1.34±0/08) were measured in T2. However, there was no significant difference between T2 and T3, in this respect (P>0.05). The numbers of red blood cells showed a significant difference between T1 (control) and three other treatments. A significant difference was observed in hemoglobin and hematocrit levels, between T1 and T2. T2 contained the highest number of white blood cells (WBC). The number of lymphocytes were increased and the number of neutrophils were decreased, with increasing the amount of Immunogen (P>0.05). Body composition analysis showed no significant difference between treatments. However, crude protein showed an increasing trend with increasing levels of probiotic. The results showed that the addition of 2 to 5 grams of Immunogen per kg diet can improve the growth performance, final production, and health (immune system) in Koi fingerlings. Therefore, due to the fact that there was no significant difference between T2 and T3 in measured parameters, 2g Immunogen per kg diet was chosen as the optimum level.