Considering the development of sturgeon farming in the country, the need to pay attention to the health of aquatic animals and the use of natural sources in fighting the pathogenicity of these fish is considered essential. In this regard, the use of medicinal plants is important for control of the disease-causing agents, including parasites, rather than chemicals. This study was conducted to evaluate the efficacy and effective dose of garlic hydroalcoholic extract in the fight against external protozoan parasite (Trichodina). For this purpose, after determination of the lethal concentrations (LC50) of this extract on Persian sturgeon, a study was conducted to determine the effective dose (EC50). In this study, the effects of garlic extract on white blood cells were also investigated. The results of differential white blood cells count in studies of EC50 extracts showed no statistically significant differences in the level of lymphocyte, monocyte and neutrophil in different treatments (P>0.05). The effective concentration of garlic hydroalcoholic extract on Trichodina sp. was determined in the concentration range of 50 to 400 mg/L and this semi effective dose was calculated as 172.5 mg/L for one hour of bath time. This dose was classified in the category of substances with low toxicity and substitution Suitable chemicals. Therefore, this extract is recommended to remove Trichodina parasite in sturgeon fish farms.