In this study, growth factors of trout fingerling fed the encapsulated arginine compared with free arginine. Therefore, about 400 rainbow trout (≈ 1 g) after adaptation to experimental environment were fed according to treatments. After assessing the shelf life of the amino acid and chitosan nanoparticles preparation, the growth was examined. Fish divided into 4 groups containing 100 individuals in pools with a separate nursery. The first group of fish were fed with commercial diet at a rate of 5% of body weight 5 times a day, the second group, fed by diets containing nanoparticles of chitosan 5% of body weight per day, the third group fed by a diet containing arginine at a rate of 5% of body weight and the fourth group were fed by a diet containing chitosan and amino acid arginine at a rate of 5% of body weight per day, for 60 consecutive days. Increasing the molecular weight of chitosan increased the size of the nanoparticles. Retention of amino acids in dietary supplements was significantly higher (P≤0.05). Most of arginine- chitosan encapsulation observed in chitosan with molecular weight of 110. Microencapsulation of arginine with chitosan resulted in more controlled amino acid release and reduced it leaching (P≤0.05). Arginine supplementation significantly improved the growth factors of Rainbow trout juveniles (P≤0.05).