The presence of contaminants in aquatic bodies causes stress responses in aquatic animals, especially fish, which ultimately affects the physiological condition of fish and reduces their immune function, so the use of immune stimulants such as prebiotics is essential. The aim of this study was to investigate the effect of different levels of pleurotus ostreatus on the histological characteristics of tilapia fish exposed to AgNO₃. This research was conducted in the fall of 1397 in the aquaculture hall of Shahid Nasser Fazli Bar Abadi, Faculty of Fisheries, Gorgan University of Agriculture and Natural Resources. 120 fish were distributed for 42 days in 4 treatments (3 replicates) prebiotic. Then, 0.5 ppm of AgNO₃ was exposed to each group for 16 days. At the end of the course, the fish were rapidly anesthetized by clove anesthetic solution (220 mg / l) and their liver tissue and gills were removed for histological studies. The results showed that the treatments that were exposed to AgNO₃ included tissue complications including basal hyperplasia, head hyperplasia, infiltration of blood cells, and shortening of the secondary blade, epithelial bulge, squamous cell swelling, and secondary bladder connective tissue involvement in gill tissue. They showed necrosis of fat, thinning, dark granules, swelling, blood clots, bleeding, and biliary stagnation in the liver tissue, and complications in this treatment had the greatest destructive effect. AgNO₃ treatments cause severe damage to liver tissue and gills, but the combined use of AgNO₃ and mushroom prebiotics has been able to reduce the destructive effects of AgNO₃ on tissue complications. 0.2 prebiotic levels of mushrooms and 0.5 AgNO₃ in the diet could have the best effect on the complications of liver tissue and Tilapia fish gills.