The effects of substituting dietary protein sources with different levels of Sargassum ilicifolium on growth, survival and body composition of rainbow trout Oncorhynchus mykiss were investigated over the course of a 60 day experiment. A total of 360 juveniles (75±2.8 g) were randomly allotted to four treatment groups including (control: with 100% basal diet (BD); T₁: 5% sargassum meal (SM)+95% BD; T₂: 7.5% (SM)+92.5% BD and T₃: 10% (SM)+90% BD. Each treatment group was divided into three replicates of 30 fish per replicate. At the end of the experiment, our results showed that replacement of 5% and 7.5 % of basal diet with sargassum meal showed significant differences in average weight and total length, feed conversion ratio (FCR), specific growth rate (SGR), weight gain percent (WG), condition factor (CF) and survival rate (SR). There were significant differences between calorie content of carcass in T₂ and control with T₁ and T₃ (p<0.05). The highest values of carcass protein content were observed in T₁ (p<0.05). There were significant differences between lipid content of fish in T₂ and control with that of fish in T₁ and T₃ (p<0.05). The highest values of carcass ash content were observed in T₁, (p<0.05) which were not significantly different from that in other treatments (p>0.05). Levels all of amino acids were higher in fish in T₂.