Probiotics are widely used in modern aquaculture. The aims of this study were to investigate the effect of probiotic bacteria of Bacillus subtilis strain on growth parameters of rainbow trout and to determine the best application rate of this bacterium. The results of this research will be widely used in improving the production of this valuable species and creating new perspectives in breeding conditions and increasing its nutritional value. In this study, the effect of Bacillus subtilis IS02 in a completely randomized design with four different treatments including: T₁ probiotic supplementation- free, T₂ containing 10⁶, T₃ with 10⁷ and T₄ with 10⁸ CFU/g of B. subtilis in diet for 30 days on some growth parameters (WG, SGR, FCR, PER, LER, PPV, LPV, CF, VFI, FE, DGI) was investigated. Data analysis was performed using one-way ANOVA and Bonferroni test. The results showed that T₄ treatment with a concentration of 10⁷ CFU/g had a significant effect (p=0.001) on the increase of protein value (PPV), but no significant effect was observed in other parameters of growth (p> 0.05).