In this study, the effect of different dietary levels of selenium on the growth of survival and muscle chemical composition of rainbow trout (Oncorhynchus mykiss) with an average weight of  80±0.57 g was investigated for 60 days. 320 fish were distributed randomly in 12 fiber tanks of 1500 L. Selenium was added at four levels, no selenium (control), 0.5 (treatment1), 1.5 (treatment2), and 3 (treatment3) mg/kilogram and the fish were fed for 6 weeks. The results showed that the specific growth rate, percentage of body weight gain, average daily growth, and food conversion ratio in the treatment fed with a diet containing 3 mg/kg selenium were significantly different from the control (p<0.05). There was no significant difference in the survival index between the investigated treatments and the control group (p>0.05). The effect of adding selenium on the chemical composition showed a significant difference in the level of carcass protein (p<0.05). The highest value related to treatment 4 (18.20±0.57 %) and the lowest value related to control group (16.47±0.16 %). In the treatments fed with selenium, a decrease in rainbowtrout fat was seen, which was significantly different from the control group (p<0.05). The moisture and ash levels in the control group were not significantly different from other treatments (p>0.05). The highest effect of selenium on growth was related to treatment 4 (3 mg/kg of selenium), but there was a significant difference with treatment 3 (1.5 mg/kg selenium) did not have (p>0.05). Overall, the results of this research showed the positive effect of adding 3 mg/kg selenium to the diet on the growth indices and improving the chemical composition of the rainbow trout.