The aim of this study was to investigate that the effect of commercial product of Multi-Behsil probiotic, including a mixture of several species of probiotic lactobacilli, bacilli, yeast and other probiotic microorganisms on growth and feeding performance in Oncorhynchus mykiss juveniles through the supplementation of diets. For this purpose, number of 600 fish juveniles with average mean weight 19.08±0.65 g were randomly introduced into 12 netting cages (size of 0.6×0.6×1 m with density of 50 fish per cage and placed in raceway concrete ponds). This experiment was conducted in a completely randomized design with a control group and three experimental treatments, M1, M2 and M3. In experimental treatments fish were fed with supplemented diets in levels of 200, 300 and 400 mg/kg of Multi-Behsil probiotic in diet for 45 days respectively. At the end of the trial, final weight and length, Average daily Growth, and Feed Conversion Efficiency showed significant differences in comparison with the control (P<0.05). There was no significant difference on specific growth rate, Thermal Growth Coefficient and Energy retention efficiency in comparison with the control group(P>0.05). While the crude protein (19.71%) and crude lipid (7.44%) of carcass in treatment of M3 had significantly increase (P<0.05) in comparison with the control group (crude protein 18.53% and crude lipid 7.17%). This study highlighted that the different levels of Multi-Behsil probiotic in rearing of Rainbow trout juveniles had different effects on growth and feeding performance in this fish.