In this study, the effect of  adding different  levels of  Montmorillonite, on growth indices and some blood factors of rainbow trout frys was investigated. Study carried out during 123 day within 3 experimental treatments,consist of adding different levels of Montmorillonite to the commercial diet with 4 replication in each treatment. In each replication 45pcs of fry with average weight of 500±0.35mlg were distributed. For this study, fiberglass rectangular tanks with 300L capacity, filled with 200L water were used. Frys were fed eight times daily. Weight measurement of frys done each 15 days and blood sampeling from caudal vein was carried out at the end of experimental period. The results showed significant improve in final weight, percentage of weight gain, daily weight gain, SGR and FCR in experimental treatments comparing control (p<0.05). The PER and HSI, showed no significant statistical difference between experimental treatments and control (p˃0.05). The highest final weight and highest percentage of final weight and the best FCR was observed in experimental treatment with 2% Montmorillonite in feed and highest daily weight gain and highest SGR was observed in experimental treatment with 4% Montmorillonite in feed. The blood factors, showed no significant difference between experimental treatments and control (p˃0.05). Based on this results, use of 2% Montmorillonite as a feed additive, in rainbow trout fry feeding, is recommended to improve growth indices without negative effect on blood factors.