To study the synergistic effects of potassium sorbate and Lactobacillus casei in rainbow trout fry (Oncorhynchus mykiss) (2.45±0.05), A test for 8 weeks using six treatments, including: Commercial diet (control), diet 107 CFU / g bacteria (treatment 1), a diet containing 0.5% of potassium sorbate (treatment 2), a diet containing 1% potassium sorbate (treatment 3), The composition of the diet with %0/5 of potassium sorbate and 107 CFU / g Lactobacillus casei (treatment 4) and the composition of the diet with 1% potassium sorbate and 107 CFU / g Lactobacillus casei (treatment 5) was carried out. Experiment was done in completely random design. The results showed that the highest final weight, SGR, FCR and PER in the treatment 4 showed significant differences with other treatments (P<0.05).The carcass significant differences were not observed between the different treatments (P>0/05). WBC was significantly higher (P<0.05) in T4 compared with other groups. No significant difference in the amount of RBC, Hb, HCT, MCV, MCH, MCHC were observed (P>0/05). The number of intestinal bacteria and Lactobacillus showed significant differences between the different treatments So that highest rate was observed in treatment 4 (P <0.05). In total, according to the results and the positive effects of the addition compound CFU / g 107 bacteria (Lactobacillus casei) and 0/5% of potassium sorbate in the diet of rainbow trout diet is recommended.