Sodium alginate is an anionic polysaccharide and a linear copolymer, whose positive functions have been proven in various fields, such as aquaculture. In this study, the effects of feed additive containing different levels of modified sodium alginate in comparison with simple alginate and commercial diet on growth factors and non-specific immune system parameters of rainbow trout (3 ± 0/3 g) in 3 replicates, reviewed for 45 day. Modified alginate supplements at levels, 1, 2.5 and 5 g/kg and simple alginate were added at a level of 2.5 g/kg to the basal diet. In general, the effects of dietary supplemented of modified alginate 5 g/kg on the final weight, weight gain, FCR and SGR were significantly increased compared with the control group (P 0.05). Significantly difference (P 0.05) was observed in non-specific immune system parameters including the serum lysozyme activities, complement activities, bacterial flora and the level of lacticacid bacteria in modified alginate 5 g/kg treat and bactericidal activities showed a significantly difference in diet containing modified alginates at 2.5 g/kg between the experimental and control treatments. The results showed that diet containing modified alginate has improved the growth performance and increased functions of the non-specific immune system parameters of rainbow trout juveniles compared to the control group. Based on the results of this study, modified alginate can be considered as beneficial feed additive and immunostimulant in Oncorhynchus mykiss juveniles.