Beluga (Huso huso) is one of the most valuable species of sturgeons and many efforts have been made to artificially reproduce, culture and restore this precious species. The objective of this study was to find the effects of garlic essential oil as an immunostimulant on hematological indices and on the cellular immunity system of juvenile belugas. An experiment was organized in which six different diets were fed to belugas including: diet without garlic essential oil and antibiotic, diet with oxytetracycline antibiotic (30 mg/kg), and four diets with various amounts of garlic essential oil (0.05, 0.10, 0.15 and 0.20 g/kg). Beluga juveniles with an average weight of 24 ± 2 g were fed these diets for 56 days. At the end of the experiment, hematological indices were determined. The blood coagulation time and mean corpuscular hemoglobin (MCH) indices increased in the groups treated with garlic essential oil. Significant increases in lymphocyte counts were observed in groups fed 0.05, 0.10 and 0.15 g/kg garlic essential oil as compared with the antibiotic and control groups. A significant decrease in the blood neutrophil count was observed in the belugas fed 0.15 and 0.20 g/kg garlic essential oil compared with the other treatments. In conclusion, these results showed that adding garlic essential oil increases the blood coagulation time, the hemoglobin content of red blood cells, lymphocyte numbers, and decreases eosinophils. Significant improvements of the immune system and of the physiological condition were observed in juvenile belugas receiving garlic essential oil compared with the control and the antibiotic treatments. It can therefore be used as a suitable replacement for oxytetracycline in the diet of juvenile belugas.